Chapter 10

Another mistake is often made in regard to the cause of certain prismatic colors that are sometimes, in a striking degree, produced by otherwise good objectives. According to the nature of these colors, whether yellow or blue, green or indigo, they are generally regarded as evidences of either chromatic over or under correction of the objective. Of course the presence of either of these defects is certainly and correctly indicated by the appearance of one or the other of the colors, under certain circumstances; but the simple visibility of prismatic color is by no means a reliable indication of over or under correction of color, and, indeed, to the honor of our opticians, it may be stated that very few objectives are made that cannot justly be called achromatic in the general sense of the term. By far the most common causes of prismatic color, in otherwise carefully constructed objectives, are the so-called chromatic aberrations of second or higher order. Every achromatic lens which is, as it should be, at its best at about two-thirds of its aperture, is inside of this ring or zone, toward the center slightly under and outside, toward the edge, slightly over corrected. This defect is the greater, the less the difference of the dispersive powers of the two glasses used in the construction of the lens, for a given proportion of their refractive indexes, and therefore the degree of visibility of the colors of the aberrations of the second order depends greatly on the nature of the glass employed in the construction of the lens.

This defect may be corrected by a suitable combination of two or more lenses, though not without again having similarly, as in the correction of the first color, some faint remnants of color, the aberrations of third or still higher order. But even the correction of the third or still higher order may, if the angular aperture is very wide, leave quite visible and disturbing remnants of color.

Another and not uncommon explanation of the cause of this unwelcome color, though not so serious and damaging a charge to the maker of the objectives, is its attribution to the so-called "secondary spectrum." This error, like that previously mentioned, is certainly indicated by the appearance of certain colors under certain conditions, but being, as a rule, one of the least defects of even our best objectives in most cases, it is probably not the true source of the disturbance.

The secondary spectrum is very commonly confounded with the chromatic aberration of higher order. While the latter is produced by imperfections in the form of the lens, the former is due to an imperfection of the optical qualities of the material from which the lens is constructed, the crown and flint glass.

A glass prism of any angle will project upon a white surface a spectrum of any length, according to the arrangement of the light source, the screen, and the prism. So with two prisms of the same kind of glass, but of different angles, two spectra can be produced of exactly equal length, so that if one is brought over the other, with the corresponding colors in line, they will appear as one spectrum. But if one of the prisms is made of crown and the other of flint glass, then their spectra cannot be arranged so that all their corresponding colors would be in line, for the proportional distances of the different colors differ in the two spectra. If two colors of the spectra are, by suitable arrangement, brought exactly in line, then the others will be out. The two spectra do not coincide, and the result, if an achromatic lens be made of these glasses, must be a remnant of color which cannot be neutralized. This remnant is the secondary spectrum.

Although this peculiar disharmony in the dispersive powers of the two glasses, crown and flint, was discovered almost immediately after achromatism was invented, it was only recently that the first successful attempts were made to produce different glasses, which, possessing the other requirements for achromatic objectives, would produce coincident spectra, or nearer so than the ordinary crown and flint glass do. It was about twelve years ago, if my memory serves me, when I learned that a well-known English firm, engaged in the manufacture of optical glass, had brought out some new glass possessed of the desired qualities, and a little later I received a circular describing the glass. But at the same time I learned that the new glass was very soft and difficult to polish, and also that it had to be protected from the atmosphere, and further, that an English optician had failed to construct an improved telescope objective from it. I had ordered some samples of the glass, but never received any.

A few months ago, news from Europe reached this country that another and seemingly more successful attempt had been made to produce glass that would leave no secondary spectrum, and that Dr. Zeiss, the famous Jena optician, had constructed some new improved objectives from it. But the somewhat meager description of these objectives, as given by an English microscopist, did not seem fit to excite much enthusiasm here as to their superiority over what had already been done in this country. Besides this, the report said that the new objectives were five system, and also that extra eye-pieces had to be used with them. I confess I am much inclined to attribute the optical improvement, which, according to Dr. Abbe's own remark, is very little, more to the fact that the objectives are five system than to the new glass used in their construction.

After a close study of a descriptive list of the new glass, received a week or two ago from the manufacturers, I find, to my great regret, that this new glass seems to suffer from a similar weakness to that made by the English firm twelve years ago; as all the numbers of the list pointed out by the makers as having a greatly reduced secondary spectrum are accompanied with the special remark "to be protected." Furthermore, from a comparison of the dispersive and refractive powers of these glasses, as given in the list, I find that objectives constructed from them will leave so great aberrations of higher order, both spherical and chromatic, that the gain by the reduction of the secondary spectrum would be greatly overbalanced.

In conclusion, I wish to say that while I would beware of underestimating the great scientific and practical value of the endeavor of the new German glass makers to produce improved optical glass, and the great benefit accruing to opticians and all others interested in the use of optical instruments, I think it wise not to overestimate the real value of the defects of the common crown and flint glass, which I have sought to explain in this paper. And, for myself, I prefer to fight the more serious defects first, and when its time has come I will see what can be done with the secondary spectrum.

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PROBABLE ISOLATION OF FLUORINE. DECOMPOSITION OF HYDROFLUORIC ACID BY AN ELECTRIC CURRENT.

By M.H. MOISSAN.

In a former memoir[1] we showed that it was possible to decompose anhydrous hydrofluoric acid by the action of an electric current. At the negative pole hydrogen collects; at the positive pole a gaseous body is disengaged, having novel properties. The experiment was performed in a platinum U tube, closed by stoppers of fluorite, and having at the upper part of each branch a small delivery tube, also of platinum. Through the stopper passes a platinum rod, which acts as electrode. The metal employed for the positive pole is an alloy containing 10 per cent. of iridium.

[Footnote 1: _Comptes Rendus_, vol. cii., p. 1543, and _Chemical News_, vol. liv., p. 36.]

To obtain pure anhydrous hydrofluoric acid, we begin by preparing fluorhydrate of fluoride of potassium, taking all the precautions pointed out by M. Fremy. When the salt is obtained pure, it is dried on a water bath at 100°, and the platinum capsule containing it is then placed in a vacuum in the presence of concentrated sulphuric acid, and two or three sticks of potash fused in a silver crucible. The acid and potash are renewed every morning for a fortnight, and the vacuum is kept at 2 cm. of mercury. Care must be taken during this desiccation to pulverize the salt every day in an iron mortar, so as to renew the surface. When the fluorhydrate contains no more water it falls to powder, and is then fit to serve for the preparation of fluoric acid; the fluorhydrate of fluoride of potassium, if well prepared, is much less deliquescent than the fluoride.

When the fluoride is quite dry, it is quickly introduced into a platinum alembic, which has just been dried by heating it to redness. The whole is kept at a gentle temperature for an hour or an hour and a half, so as to allow the decomposition to commence very slowly; the first portions of acid which come over are rejected as they carry with them traces of water remaining in the salt. The platinum receiver is then attached, and the heat increased, allowing the decomposition to proceed with a certain degree of slowness. The receiver is then surrounded with a mixture of ice and salt, and from this moment all the hydrofluoric acid is condensed as a limpid liquid, boiling at 19.5°, very hygroscopic, and, as is well known, giving abundant fumes in presence of the atmospheric moisture.

During this operation the platinum U tube, dried with the greatest care, has been fixed with a cork in a cylindrical glass vessel surrounded with chloride of methyl. Up to the moment of introducing the hydrofluoric acid, the leading tubes are attached to drying tubes containing fused caustic potash. To introduce the hydrochloric acid into the apparatus, it may be absorbed through one of the lateral tubes in the receiver in which it is condensed.

In some experiments we have directly condensed the hydrofluoric acid in the U tube surrounded with chloride of methyl; but in this case care must be taken that the tubes are not clogged up by small quantities of fluoride carried over, which would infallibly lead to an explosion and projections, which are always dangerous with so corrosive a liquid.

When we have introduced in advance in the small platinum apparatus a determined amount of hydrofluoric acid cooled with chloride of methyl, in tranquil ebullition at a temperature of -23°, the current of 20 cells of Bunsen large size, arranged in series, is passed through by means of the electrodes. An amperemeter in the circuit admits of the intensity of the current being observed.

If the hydrofluoric acid contains a small quantity of water, either by accident or design, there is always disengaged at the positive pole ozone, which has no action on crystallized silicium. In proportion as the water contained in the acid is thus decomposed, it is seen by the amperemeter that the conductivity of the liquid rapidly decreases. With absolutely anhydrous hydrofluoric acid the current will no longer pass. In many of our experiments we have succeeded in obtaining an acid so anhydrous that a current of 25 amperes was entirely arrested.

To render the liquid conducting, we have added before each experiment a small quantity of dried and fused fluorhydrate of fluoride of potassium. In this case, decomposition proceeds in a continuous manner; we obtain at the negative pole hydrogen, and at the positive pole a regular disengagement of a colorless gas in which crystallized silicium in the cold burns with great brilliancy, becoming fluoride of silicium. This latter gas has been collected over mercury, and accurately characterized.

Deville's adamantine boron burns in the same manner, but with more difficulty, becoming fluoride or boron. The small quantity of carbon and aluminum which it contains impedes the combination. Arsenic and antimony in powder combine with this gaseous body with incandescence. Sulphur takes fire in it, and iodine combines with a pale flame, losing its color. We have already remarked that it decomposes cold water, producing ozone and hydrofluoric acid.

The metals are attacked with much less energy. This is due, we think, to the small quantity of metallic fluoride formed preventing the action being very deep. Iron and manganese in powder, slightly heated, burn with sparks. Organic bodies are violently attacked. A piece of cork placed near the end of the platinum tube, where the gas is evolved, immediately carbonizes and inflames. Alcohol, ether, benzol, spirit of turpentine, and petroleum take fire on contact.

The gas evolved at the negative pole is hydrogen, burning with a pale flame, and producing none of these reactions.

When the experiment has lasted several hours, and there is not enough hydrofluoric acid left at the bottom of the tube to separate the two gases, they recombine in the apparatus in the cold, with violent detonation.

We have satisfied ourselves, by direct experiment, that a mixture of ozone and hydrofluoric acid produces none of the reactions described above.

It is the same with gaseous hydrofluoric acid. Finally we may add that the hydrofluoric acid employed, as well as the hydrofluorate of fluoride, were absolutely free from chlorine.

The gas obtained in our experiments is therefore either fluorine or a perfluoride of hydrogen.

New experiments are necessary to settle this last point. We hope soon to lay the results before the Academy.--_Comptes Mendus_, vol. ciii., p. 202, July 19, 1886; _Chem. News._

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COHESION AND COHESION FIGURES.[1]

[Footnote 1: Notes from a lecture given to the Halifax Scientific Society, July 19, 1886.]

By WILLIAM ACKROYD, F.I.C.

_1. A Law of Solubility._

It is customary to regard cohesion as the force which binds together molecules of the same substance, and in virtue of which the particles of solids and liquids are kept together, and also to speak of the attraction exerted between particles of two different bodies as adhesion. The distinction between cohesion and adhesion is a conventional one. The similarity, if not identity, of the two forces is demonstrated by the fact that while cohesion is exerted between particles of the _same_ body, adhesion is exerted with most force between particles of _allied_ bodies. Generally speaking, organic bodies require organic solvents; inorganic bodies, inorganic solvents. For example, common salt is highly soluble in water, but not in ether, and many fats are soluble in ether, but not in water. So many cases like these will suggest themselves to the chemist that I am justified in making the following generalization: _A body will dissolve in a solvent to which it is allied more readily than in one to which it in highly dissimilar._ Exceptions to the law undoubtedly exist, but none so striking as the following in support of it, viz., that the metal mercury is the only known true solvent for many metals at the normal temperature.

_2. Its Connection with Mendeleeff's Periodic Law._

From this standpoint the whole subject of solution is deserving of fresh attention, as it appears highly probable that, just as Prof. Carnelley has shown by the use of my meta-chromatic scale, the colors of chemical compounds come under definite laws, which he has discovered and formulated in connection with Mendeleeff and Newlaud's periodic law,[2] so, likewise, may the solubility of an allied group of compounds, in regard to any given solvent under constant conditions of temperature, conform to similar laws; that, e.g., the chlorides of H, Na, Cu, and Ag, in Mendeleeff's Group I., may vary in their solubility in water from an extreme of high solubility in the case of hydrogen chloride to the opposite extreme of comparative insolubility in the case of silver chloride. In this natural series of compounds, hydrogen chloride is the body nearest akin to water, and silver chloride the most remote in kinship.

[Footnote 2: _Philosophical Magazine_, August, 1884.]

_3. A Solidified Vortex Ring._

It is in virtue of cohesion that a freely suspended drop of liquid assumes the spherical form. If such a sphere be dropped on to the surface of a liquid of higher specific gravity at rest, one obtains what is called the cohesion figure of the substance of the drop. A drop of oil, e.g., spreads out on the surface of water until it is a circular thin film of concentric rings of different degrees of thickness, each displaying the characteristic colors of thin plates. The tenuity of the film increases; its cohesion is overcome; lakelets are formed, and they merge into each other. The disintegrated portions of the film now thicken, the colors vanish, and only islets of oil remain. Some liquid drops of the same or higher sp. gr. than water do not spread out in this fashion, but descend below the surface of the liquid, and, in descending, assume a ring shape, which gradually spreads out and breaks up into lesser rings. Such figures have been termed submergence cohesion figures; they are vortex rings. I have solidified such vortex rings in their first stage of formation. If drops of melted sulphur, at a temperature above that of the viscous state, be let fall into water, the drops will be solidified in the effort to form the ring, and the circular button, thick in the rim and thin in the center, may be regarded as a solidified vortex ring of plastic sulphur.

_4. That a Submergence Cohesion Figure is a Vortex Ring._

It may be shown that the conditions of the formation of a submergence cohesion figure are those which exist in the formation of an aerial vortex. Those conditions in their greatest perfection are (1) a spherical envelope of a different nature from the medium in which the rings are produced; (2) a circular orifice opening into the medium; and (3) a percussive impact on the part of the sphere opposite the orifice. In the production of vortex rings of phosphorus pentoxide in the making of phosphoreted hydrogen, the spherical envelope is water, the orifice the portion of the bubble which opens into the air immediately it rises to the surface, and the impact is furnished by gravity. So, also, in the case of a submergence cohesion figure, the spherical envelope is the air surrounding the drop, the orifice the portion of it which first comes in contact with the liquid at rest; and here again the impact is due to gravity more directly than in the former case. These conditions are somewhat imperfectly copied in the ordinary vortex box, which is usually cubical in form, with a circular orifice in one side, and a covering of canvas on the opposite one, which is hit with the fist.

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[AMERICAN CHEMICAL JOURNAL.]

THE DETERMINATION OF NITRIC ACID BY THE ABSORPTION OF NITRIC OXIDE IN A STANDARD SOLUTION OF PERMANGANATE OF POTASSIUM.

By H.N. MORSE and A.F. LINN.

The method which we propose consists in the conversion of the nitric acid into nitric oxide; the absorption of the latter in a measured, but excessive, quantity of a standard solution of permanganate of potassium; and the subsequent determination of the excess of the permanganate by means of a standard solution of oxalic acid or sulphate of manganese.

THE APPARATUS.

A is an apparatus for the generation of carbon dioxide free from air, which will be explained hereafter.

B is a flask, having a capacity of 125 or 150 c.c., in which the nitrate is decomposed in the usual manner by means of ferrous chloride and hydrochloric acid.

C is a small tube for the condensation of the aqueous hydrochloric acid which distills over from B.

D is a Geissler bulb, containing a concentrated solution of potassium carbonate, to arrest any acid vapors coming from C.

E, E are two pieces of ordinary combustion tubing, having a length of about 650 or 700 mm., in which is placed the permanganate solution employed for the absorption of the nitric oxide. Their open ends are provided with lips in order to facilitate the pouring of liquids from them, care being taken not to so distort the ends that rubber stoppers cannot be made to fit them tightly. They are placed in a nearly horizontal position in order to diminish the pressure required to force the gases through the apparatus and thus lessen the danger of leakage through the rubber joints.

_a_ is a tube through which the ferrous chloride and hydrochloric acid are introduced into B, as in the method of Tiemann-Schulze.

_b_ serves for the introduction of carbon dioxide to expel the air before the decomposition of the nitrate, and the nitric oxide afterward.

_c_ is an unbroken tube ending at the lower surface of the stopper in B, and at the bottom of C.

The rubber joint, _d_, is furnished with a Mohr and also a screw pinch cock. The joints, _e_ and _f_, are furnished with Mohr pinch cocks. The rubber tubing upon these should be of the best quality, and must be carefully tied.

THE SOLUTIONS.

In consequence of the large volume of the permanganate solution required for the complete absorption of the nitric oxide, we have found it advantageous to use three solutions instead of two.

1. A solution of permanganate such that one c.c. is equivalent to about fifteen milligrammes of nitrate of potassium, according to the reaction:

KMnO_{4} + NO = KNO_{3} + MnO_{2}.

This solution is employed for the absorption of the nitric oxide. Its strength need not be exactly known. There is no objection to a more concentrated solution, except that which pertains to all strong standard solutions, namely, that a small error in measurement would then give a larger error in the results. 100 c.c. of this solution are required for each determination, and the measurement is always made in one and the same 100 c.c. measuring flask, which, if necessary, should be labeled to distinguish it from that used for solution No. 2.

2. A solution of oxalic acid which is very slightly stronger than that of the permanganate just described--that is, a solution such that one c.c. of it will somewhat more than decompose one c.c. of the permanganate, according to the reaction:

2KMnO_{4} + 3H_{2}SO_{4} + 5C_{2}H_{2}O_{4}.2H_{2}O = K_{2}SO_{4} + 2MnSO_{4} + 18H_{2}O + 10CO_{2}.

The exact strength of this solution need not be known, since we only require the difference in value between it and solution No. 1, which is determined by means of solution No. 3. 100 c.c. of this solution are also required for each determination, and the measurement, as in the preceding case, is always made in the same 100 c.c. measuring flask.

3. A dilute, carefully standardized solution of permanganate of potassium.

The method of using these solutions is as follows: 100 c.c. of No. 1 and No. 2 are measured off (each solution in its own measuring flask), brought together in a covered beaker glass, and acidified with dilute sulphuric acid. The excess of oxalic acid is then determined by means of solution No. 3.