CHAPTER VII

ROSCOE AS AN INVESTIGATOR

The character of Roscoe’s scientific work may also be said to have been entirely moulded by his Heidelberg training, and Bunsen’s influence may be traced through it to the last. So completely was this the case that consciously or unconsciously he seemed never to contemplate attacking any problem that would not have appealed to, or have been appreciated by, Bunsen.

His first research was undoubtedly suggested by Bunsen. As already stated, it resulted in the classical investigation on the laws regulating photochemical action. It was already known that a mixture of equal volumes of hydrogen and chlorine on exposure to light lost its characteristic colour, and was converted into hydrochloric acid, readily soluble in water; and Bunsen conceived the idea of making this reaction the basis of a method of measuring the relative amount and activity of those light-vibrations which are mainly concerned in effecting chemical change. As a matter of fact the idea was not new, for, unknown to Bunsen, it had already been adopted by Draper, of New York, who had, as he states in his paper in the _Philosophical Magazine_ for December 1843:

invented an instrument [based upon the same reaction] for measuring the chemical force of the tithonic rays, which are found at a maximum in the indigo space, and which from that point gradually fade away to each end of the spectrum.

It perhaps says little for the assiduity with which young Roscoe read the original chemical literature of his time that he should only have knowledge of Draper’s remarkable papers some thirteen years after they were published in the English journals. But it is only due to him to say the chemical students of University College in those far-off days had fewer opportunities of access to original literature than they now enjoy.

Be this as it may, Roscoe’s discomfiture at being thus anticipated was of no long duration.

Do not (wrote Bunsen) let your discovery of Draper’s work disconcert you.… It appears to me that the value of an investigation is not to be measured by whether something is described in it for the first time, but rather by what means and methods a fact is proved beyond doubt or cavil, and in this respect I think that Draper has left plenty for us to do.

After many fruitless attempts they succeeded in constructing an apparatus in which the defects of Draper’s “tithonometer” were obviated, and by which not only accurate comparative determinations could be made, but which enabled them to reduce the chemical action of light to absolute measure. They showed by means of it that the amount of chemical action produced by light from a constant source varied inversely as the square of the distance. They studied more accurately the phenomena of photochemical induction, discovered by Draper, the causes which determine its occurrence, and the laws which regulate the chemical action of light after the induction is completed. They proved that the absorption of the chemical rays in passing through a medium varies directly as the intensity of the light, and that the amount transmitted varies proportionately with the density of the absorbing medium. It was found that for a given amount of chemical action effected in the mixture of chlorine and hydrogen an equivalent quantity of light is absorbed, and that the coefficients of extinction of pure chlorine for chemical rays from various sources of light are very different. They established a general and absolute standard of comparison for the chemical action of light, and sought to determine the quantitative relations of the chemical action effected by direct and diffused sunlight, and to investigate the laws which regulate the distribution on the earth’s surface of the chemical activity emanating from the sun. They also measured the chemical action of the constituent parts of the solar spectrum. The action on the sensitive gas showed the existence of several maxima of chemical intensity in the spectrum. The greatest action was observed between the lines G in the indigo and H in the violet, whilst another maximum was found to be near the line I in the ultra violet. Towards the least refrangible end of the spectrum the action became imperceptible about the line D in the orange, but at the other end of the spectrum the action was found to extend as far as Stokes’s line U, or to a distance from the line H greater than the total length of the ordinary visible spectrum.

By investigating the conditions under which it was possible to prepare a photographic paper of uniform and constant sensitiveness, and ascertaining the means by which the darkening of the paper on insolation could be accurately compared with a standard tint, it was found comparatively easy to construct an instrument capable of measuring the chemical action of light effected at any point on the earth’s surface by the total sunlight and diffuse daylight under the most widely varying circumstances of climate and atmospheric condition.

This joint research, begun in 1855, occupied its authors until 1862. Roscoe did the greater part of the experimental work, and after his election to the professorship in Owens College in 1857 he spent his long vacations in Heidelberg in continuing the inquiry. The results were communicated in a series of memoirs to the Royal Society and are published in the _Philosophical Transactions_.[5]

Subsequently he pursued the subject alone or in conjunction with others. In a short paper published in 1863 he gave the results of a series of measurements of the chemical brightness of various portions of the solar disc made by means of standard photographic paper according to the method described by Bunsen and himself in their last communication;[6] and in 1864 he described a method of meteorological registration of the chemical action of total daylight based on a modification of that originally used by Bunsen and himself. The account of this method was made the Bakerian Lecture in 1865, and is published in the _Philosophical Transactions_ of that year.[7]

In this paper he gives the results of consecutive observations on each day for nearly a month at about midsummer, and compares the chemical action of light at Manchester at the winter and summer solstices, and the vernal and autumnal equinoxes. The wide variation in the chemical action of light at different periods of the year was illustrated by the fact that if the integral of that on the shortest day be taken as the unit, that upon the equinox will be represented by 7, and that upon the longest day by 25.

In 1866 he and Mr. Baxendell contributed a joint note to the Royal Society on the relative chemical intensities of direct sunlight and diffuse daylight at different altitudes of the sun. They showed from observations made at Manchester and at Heidelberg that the ratio of the chemical intensity of direct to diffuse sunlight for a given altitude at different localities is not constant, but varies with the transparency of the atmosphere, and that this ratio does not in the least correspond with the value of visible intensity as estimated by the eye, the action of the atmosphere being 17·4 times greater upon the chemical than on the luminous rays when the sun’s altitude is about 25° and 26·4 times greater when it is 12°.[8]

With a view to the introduction of the instrument into meteorology, and as part of the routine work of an observatory, he caused a regular series of measurements to be made during two years at the Kew Observatory, under the direction of Dr. Balfour Stewart; and in order to gain further knowledge of the variation in the chemical action of light in different areas of the earth’s surface, he sent the writer in 1866 to Pará, on the river Amazon, 1° 28´ south of the Equator. The selection of this particular place arose from the circumstance that his cousin, now the Right Hon. Charles Booth, was proceeding to the Brazils in connection with the establishment of a new line of steamers, and it was arranged that the writer should accompany him, in order that he might make photometric observations _en route_, going and returning, and at the same time make a series of determinations of the amount of carbon dioxide in sea air, night and day, with a view of testing the truth of an allegation by a French chemist, that it was subject to a diurnal variation, depending upon the intensity of sunlight.

The special form of apparatus arranged for making photometric observations at sea proved to be ill-adapted to the purpose. But even if its performance had been good, the conditions under which it had to be employed were incapable of furnishing valid results. Accordingly the writer elected to remain at Pará in order to obtain the required observations at that place, until such time as he could return by the succeeding steamer. He was thus enabled to make a much more extensive set of observations than was originally contemplated, and under conditions which ensured trustworthy measurements.

The following letter, dated May 12, 1866, and received at Pará, refers to these matters. The allusion to Agassiz arose from the circumstance that the great naturalist was at the time engaged in work on the Amazons and its tributaries under the auspices of the Emperor Dom Pedro.

Roscoe himself was busily engaged with his vanadium researches.

Although I was disappointed to find that the _Augustine_ returned without you, yet I think that you acted quite rightly in staying until the _Jerome_ returns, as you could not get any results whilst on board. I only hope that your health will have been good, and that you will have enjoyed your stay at Pará, and that when you return in August we may have to work out plenty of interesting results.

I send per the _Jerome_ a second bottle of silver solution, and some more salted paper, as you may possibly be out of both. I also enclose two or three fixed strips, but _not calibrated_. These, in case you are out, may be used, _carefully preserved_ and calibrated, on your return. They must be carefully marked before using and notice taken in the book of the marks on each strip when employed.

…

Your carbonic acid observations are _very_ interesting: you seem to have settled Lewy completely, and I hope you will get some more experiments made on your return voyage.

Could you not manage to make several series of _daily_ observations (photochemical) on your return voyage? It would not much matter if the surface was not perfectly horizontal always, and you could use your pendulum concern to steady the exposed paper to a certain extent. Perhaps your ship does not give you a sufficiently free horizon. However, you will do what is possible.

Your meeting with Agassiz was very fortunate, and I was glad to hear that the other friends whom you found were likely to prove agreeable.

We have not much news to send you. The book [“Lessons in Elementary Chemistry”] is not yet out. I have this day, however, corrected the last proofs of the Index, and I fully expect that it will be ready (and I hope a great number sold!) before you arrive here.

My new assistant [Mr. Francis Jones] is a very careful and accurate worker. He has with great care determined the atomic weight of vanadium by the loss of weight of VO₃ in hydrogen, and curiously enough gets _exactly_ Berzelius’s number of 68·5! This in two experiments on large quantities. We are now preparing pure chloride to try again whether we get 67·4 (your number), and we have got hold of some very queer reactions, which I can only understand either by the presence of another metal having the same (or nearly) atomic weight as Va, or else by the existence of an isomeric (solid) modification of the chloride. However, time, I hope, will show.

My lecture on June 1st at the Royal Institution will, of course, be shorn of some of its interest as I cannot tell them how much chemical light there is in the tropics, but I hope to have enough to make an interesting hour, and have got some nice experiments to illustrate the opalescence of the atmosphere.

My wife tells me to remind you to be so good as to bring her something tropical—some birds’ skins would do or anything you see or fancy. Only no monkeys, if you please, for me! I hate the animals.

Your father will doubtless send you all the news of the times, as well as local information, and so I will only add that the Laboratory [rowing] crew were again beaten last week. The races were very good—better as regards equality than last year. The umpire, poor R⸺, got upset and nearly suffocated in the Irwell mud!

Hoping to see you safe back at the end of July or beginning of August.

The day after this letter was written Roscoe received some of the preliminary results of the photometric observations, so that he was enabled to give his Royal Institution audience some idea of the amount of chemical light in the tropics. He made this matter one of the chief features of his lecture.

I have only just time to send what you want over to Liverpool this afternoon, and to acknowledge your letter of April 14th with enclosures, which are all very welcome.

…

The diffused and direct sun experiments are very interesting. They differ _in toto_ from the Heidelberg results. Pray get some more at low elevations of the sun.

I must now close … as the _Jerome_ sails early in the morning.

P.S.—I hope you may be able to get _one_ cloudless day before you leave, as the clouds evidently much modify the result. It is almost a pity that you did not go out in the vacation for September and October, but it cannot be helped now.

If you get a cloudless day begin early, take four or six sets of observations (one at noon, of course) until late in the evening, so as to get the _low_ elevations.

The Kew observations showed that the mean chemical intensity for hours equidistant from noon is practically the same on the same day, and that the daily maximum of chemical intensity corresponds with the maximum of solar altitude. Measurements showing the daily rise and fall of chemical intensity for each of the twenty-four months were obtained, as well as of the biennial variation for the same period. It was pointed out that the curve of yearly chemical intensity is not symmetrical about the vernal and autumnal equinoxes. Thus for 100 chemically active rays falling at the spring equinox at Kew, there fell at the autumnal equinox 167 rays, the sun’s mean altitude being the same, the difference being probably due to the greater atmospheric opalescence in the spring.

The Pará observations were interesting from the fact that they were the first measurements of photometric intensity made within the tropics, and that they served to dispel certain fallacies about photographic effects in very hot climates at that time current. The observations showed that the relation between the sun’s altitude and chemical intensity may be represented by the equation:

C I _a_ = C I₀ + const. _a_,

where C I _a_ represents the chemical intensity at a given altitude _a_ in circular measure, C I₀ the chemical intensity at the altitude 0, and const. _a_ a number to be calculated from the measurements. Comparisons between the observations at Kew and at Pará on the same days in April showed that the daily mean chemical intensity at the latter place was from ten to fifty times greater than at Kew, the wide differences being due to the enormous and rapid variations in intensity from hour to hour which the chemically active rays experience in the tropics during the rainy season of the year.[9]

The relation between the sun’s altitude and the chemical intensity of daylight was more accurately determined by the writer from a long series of observations made by Roscoe’s method under a cloudless sky near Lisbon in the autumn of 1867. The fact was confirmed that the direct sunlight is robbed of almost all its chemically active rays at altitudes below 10°, and that although the chemical intensity for the same altitude at different places and at different times of the year varies according to the varying transparency of the atmosphere, yet the relation at the same place between altitude and intensity is always represented by a straight line. The differences in the observed actions for equal altitudes, which may amount to more than 100 per cent. at different places, and to nearly as much at the same place at different times of the year, serve as exact measurements of the varying transparency of the atmosphere. As illustrating the wide differences in the daily march of chemical intensity at various places, it was found that, when light of unit intensity acting for 24 hours is taken as 1,000, the value of the mean chemical intensity at Kew is represented by the number 94·5, that at Lisbon by 110, and that at Pará by 313·3.[10]

Roscoe’s hope that measurements of the chemical intensity of daylight might become part of the regular work of meteorological observations has, unfortunately, not been realized. Observations of the kind, no doubt, consume much time, and if properly conducted require the whole service of a skilled assistant. But considering the enormously important part played by chemically active light in the economy of nature, and more particularly in the phenomena of vegetable life, it cannot be doubted that a sufficiently long-continued series of observations, systematically carried out on a well-considered plan, at observatories distributed over the earth’s surface, would afford most valuable information concerning the facts of solar energy, and incidentally serve to elucidate many important collateral questions. With the assistance of Mr. Horace Darwin, Roscoe made attempts to devise an automatic arrangement which should minimize the labour of observation, but in the absence of any certain assurance that such an instrument would be utilized, the trials were discontinued.[11]

It has been thought desirable, for the sake of continuity, to describe Roscoe’s work on chemical photometry, arising out of his association with Bunsen, so long as he continued to pursue that subject. A subsequent paper will, however, be mentioned later.

We must now revert to his work when he returned from Germany.

On leaving Heidelberg to settle again in London, as already stated he engaged Dittmar as research assistant, and they jointly studied, by Bunsen’s methods, the absorption of hydrochloric acid and ammonia in water,[12] proving that these gases do not obey Dalton and Henry’s law.

He next attacked, first with Dittmar’s and then with Schorlemmer’s assistance, the nature of the aqueous solutions of the common volatile acids of constant boiling-point, and showed that although the ratio of acid to water is constant for a definite boiling-point under a particular pressure, this does not necessarily indicate the existence of definite hydrates. The composition of the hydrated acid on boiling is entirely dependent on the pressure under which it is heated—a strong solution losing acid, and a weak solution losing water until the residue in each case acquires a constant composition, depending upon the pressure under which it is boiled.[13]

In those days Gmelin’s “Handbuch” was the chief repository of chemical knowledge—or the want of it—and many suggestions as to possible profitable fields of inquiry were to be gleaned from its pages. One such subject was perchloric acid and its compounds, concerning which but little was then known beyond the composition of potassium perchlorate, established by Stadion as far back as 1816. Roscoe, with Schorlemmer’s assistance, made a fairly complete investigation of perchloric acid and its hydrates, and a number of its salts.[14]

He narrowly escaped a serious accident when working with ethyl perchlorate, first prepared in 1840 by the American chemists Hare and Boye, and known to be extremely unstable. He was engaged in filtering a few cubic centimetres of the liquid into a test-tube, when the compound exploded with great violence, and a deep hole was bored into the base of the filter-stand, and many hundreds of fragments of glass were driven into his hand. That filter-stand was long an object of interest to visitors in the private laboratory of the old Owens College.[15]

The writer subsequently prepared thallium perchlorate for him in a pure state, determined its composition, and established its isomorphism with the alkaline perchlorates, the crystallographic characters of which had been previously ascertained by Kopp.[16]

But Roscoe’s most important contribution to inorganic chemistry was unquestionably his research upon vanadium and its compounds, which occupied him for the greater part of five years. About 1865 his attention was drawn to the occurrence of vanadium in some of the copper-bearing beds of the Lower Keuper Sandstone of the Trias which were then being worked at Alderley Edge and Mottram St. Andrews in Cheshire. He obtained possession of a large quantity of a lime precipitate, which was found to contain about 2 per cent. of vanadic acid. It was a most unpromising material, but eventually a method was worked out by which the vanadium was extracted as an ammonium vanadate: this on heating yielded vanadic acid. Great difficulty was met with in freeing the vanadic acid from accompanying phosphoric acid. Even small quantities of phosphoric acid cause the vanadic acid after fusion to solidify as a pitch-like amorphous mass. It was the writer’s privilege to assist in the early stages of this investigation, and it fell to his lot to carry out the various experiments which eventually served to establish the composition of the oxides of vanadium, the true nature of its volatile chloride, the existence of hitherto unknown oxychlorides, and of the mononitride which Berzelius had regarded as the metal, and lastly to fix its real atomic weight and to show that it was approximately 16 below that assumed by Berzelius. It was only very gradually that the true chemical relationships of vanadium revealed themselves. For a time the indications were contradictory and perplexing. The first clue was given by Rammelsberg’s observation that vanadinite is isomorphous with pyromorphite and mimetesite—analogously constituted minerals containing phosphorus and arsenic. The next significant fact to be discovered was that by the action of a reducing agent it was possible to obtain a solution of a vanadium oxide which on reoxidation to vanadic acid appeared to require as much oxygen as Berzelius’s vanadium, regarded as metal, would have needed. When it was discovered that the volatile chloride which Berzelius had considered was a trichloride and free from oxygen, in reality contained oxygen, and was analogous in constitution to phosphoryl chloride, the whole matter was rapidly cleared up, and the chemical affinities of vanadium to phosphorus, arsenic, and the other members of the trivalent group were established. This, of course, necessitated altering the formulæ of the vanadium compounds hitherto described.

At the close of the College Session 1866-1867, Roscoe took away with him the laboratory journals containing the results of the inquiry as far as it had progressed, and worked at them at Roddam Hall, near Alnwick, which he had taken for the Long Vacation.

The following letters have reference to this matter:

RODDAM, NEAR ALNWICK, _August 26, 1867_.

I write a line to say that I hope you are getting on well and that I shall soon hear from you.… I want you very much to stay with me till April to settle the vanadium and light matters and help me in London with my lectures.… I have at last found out about vanadium. The acid is V₂O₅ like P₂O₅. The chloride VOCl₃ like POCl₃ and the solid chlorides VOCl₂, VOCl, etc. This explains the isomorphism of the vanadate of lead and the corresponding phosphate and lots of other points. It becomes very interesting now.

Pray write a line and say whether you will stay till April, and when you will be back.

The first paragraph in the next letter alludes to the circumstance that the present writer had just returned from Lisbon, where he had carried out the photometric measurements already referred to.

RODDAM HALL, NEAR ALNWICK, _September 12, 1867_.

I was very glad to hear by your note received to-day of your safe arrival and the success of your observations.… You did quite right in returning home rather than wait indefinitely for the _Jerome_. The working out of your Pará and of the Lisbon direct and diffuse-light experiments will take some time and labour, but I believe the results will repay the trouble.…

I have been up to the Dundee [British Association] meeting for a few days, but I now, in all probability, shall stop here, so that I can at once answer your letter.

Please ask Joseph [Heywood] to send me per book-post _Pogg. Ann._, vol. 98, in which volume is Rammelsberg’s paper on the isomorphism of vanadates and phosphates. There is no doubt in my mind that vanadic acid is V₂O₅, and it will be _exceedingly_ interesting to work out the vanadates which must all be explained as phosphates. The ordinary white NH₃ salt is NH₄VO₃ (like NaPO₃) and is a meta-vanadate. The bi-vanadates can also be explained, but all need re-preparation and analysis. Did I tell you that we have now got

V₂O₅, V₂O₄, V₂O₃, V₂O₂ (I wish we had V also!) V₂O₂Cl₆, V₂O₂Cl₄, V₂O₂Cl₂, and V₂O₂Cl₆, or VOCl₃, VOCl₂, VOCl

At St. Andrews I saw Professor Heddle; he has a crystal half apatite and half vanadinite, and he threw out the suggestion long ago that vanadic acid is V₂O₅.… I hope you will write soon and let me hear what you have to say about my plan. I will then write what I think of your ideas.

* * * * *

RODDAM, _Tuesday, September 17, 1867_.

I have now edited the atomic weight determinations by oxidation, and also the various oxides of vanadium. I have now to do the chlorides. Many points still remain requiring clearing up.

(1) As regards the slow oxidation of the V₂O₃ (Berzelius’s suboxide); a sample made on November 13, 1866, weighing 0·7507, weighed on June 12, 1867, 0·8733. This corresponds to an oxide higher than V₂O₄ (V = 51·4). Now I want this oxide and tube drying under the air-pump and weighing carefully and keeping for further examination (the weight of tube and oxide was 3·6066 on June 12th last).

(2) When a neutral solution of V₂O₂ in SO₄H₂, got by zinc reduction, and neutralized by excess of zinc, is exposed to a current of air, it goes brown, and this brown colour does not alter. Analysed by permanganate it would seem to contain V₂O₄. I want this confirming.

(3) Try to get sight of a copy of Greg and Lettsom’s “British Mineralogy” (you can go and call on Mr. Robt. P. Greg, Greg Bros., Chancery Lane, and ask him to lend you the book for me). Under “Vanadinite” you will see some mention of Heddle and his observations. I find nothing about him in Rammelsberg’s paper.

(4) Has V₂O₄ been prepared by heating the suboxide V₂O₃ in the air at low temperatures? Is it green?

The thing above all others necessary for us now is to get the _metal_. We must set about this at the beginning of the session.

If you have time before you go away to clear up these four preliminary points I shall be glad.

* * * * *

RODDAM, _September 26, 1867_.

The first thing to do when we begin will be to try to get the metal V = 51·3 by forming the ammonio-chloride and reducing the nitride in ammonia.… If we only can get 0·5 of metal the question of atomic weight is settled beyond dispute.

I have now collated all the experiments: (1) On the oxides of vanadium; (2) on the determination of atomic weight by reduction of V₂O₅ to V₂O₃; (3) on the oxychlorides and determination of atomic weight from the chlorine determinations. These latter require re-calculation as I do not know how much should be allowed for as impurity in the silver. Still, I see already that this alteration can be but very slight, and the numbers agree very well, viz. (1) mean at. wt. from reduction expts. 51·362 (probable error 0·068); (2) mean of 9 volumetric chlorine determinations of the oxytrichloride 61·28; (3) mean of 8 weight determinations of chlorine 61·21, so that as far as I yet see the true atomic weight is 51·30.

I am obliged by your extract from Greg and Lettsom. When in St. Andrews I saw Professor Heddle, and he promised to send me some crystallized specimens of vanadinite and pyromorphite existing in one crystal.

I think I have got quite matter enough for one paper, but I should like to have the metal if only in sufficient quantity to oxidize up to V₂O₅ and determine the increase. Everything can be very nicely used, and all fits in well; but, of course, such a first paper must in some points be imperfect.

I think it is perhaps best that ⸺ should give the lectures, as I am sure to want you in London several Saturdays—otherwise I should have been very glad for you to have taken the course.

You will be back on Saturday, October 5th, I suppose. We must re-calculate all the analyses with the exact atomic weight. This can soon be done, and I should like to get this first part off my hands before long.

I hope you are enjoying this splendid autumn weather.

…

As regards the blue oxide got by gradual oxidation of V₂O₃ (suboxide) we have analyses proving it to be V₂O₄—by oxidation. It is very possible that the further increase in weight is due to the hydration of this oxide. We must wait until the green substance remains constant, and we must then determine the water and the V₂O₅.

Roscoe’s first memoir on the subject was read to the Royal Society on December 19, 1867, and was made the Bakerian Lecture of that session.[17] On February 14, 1868, he gave a Friday evening discourse at the Royal Institution “On Vanadium, one of the Trivalent Groups of Elements,” when the writer acted as his lecture-assistant. Having arranged the experimental illustrations, the assistant spent a spare half-hour in wandering through the old laboratories in the cellars of the Institution, sacred to the genius and labours of Davy and Faraday. In looking over some specimens in a cupboard he came upon a small bottle containing ammonium vanadate, labelled “Sent to me by Berzelius. 1831,” and on it Faraday’s well-known monogram by way of signature. A portion of the substance was afterwards placed at Roscoe’s disposal by the late Sir Edward Frankland, at the time Fullerian Professor of Chemistry. On examination it was found to contain considerable quantities of phosphoric acid, thus serving to indicate the probable cause of the discrepancy between the numbers obtained by Berzelius and Roscoe in the course of the atomic weight determinations. It had been observed that the presence of even traces of phosphorus prevents the complete reduction in hydrogen of the vanadium pentoxide to vanadous oxide.

Some little time after the appearance of the first memoir on vanadium, the writer proceeded to Heidelberg to study under Bunsen, to whom at that time practically all Roscoe’s senior students who were in a position to go to Germany were sent. It had been reported in a French periodical on popular science that Roscoe had been awarded the Copley medal for his work on vanadium, and of course his former assistant had hastened to congratulate him on that event.

CAMFIELD PLACE, HATFIELD, HERTS, _September 13, 1868_.

In the first place let me thank you for your letter and congratulations upon the great French discovery! Many of these Parisian wonders have after all turned out myths—and this last is, I believe, no exception—the expression “Medaille de Copley” is, so far as I am aware, the French (and bad French too!) for the “Bakerian Lecture.” I am, however, none the less obliged to you for your good wishes on this occasion, and for all the valuable help which in many ways you gave me.

…

Thanks for your news of my dear friend. I have been very remiss in not writing to him. Tell him so, please; and ask him to send me the first proof-sheets of his “Filtration” paper for me to translate; unless, indeed, you do it yourself, as I am sure you can perfectly well. Give B[unsen] my kindest regards, and say that I will write to him soon.