Chapter 1

Hoffman, in one of his observations, gives the history of a powder called _magnesia alba_, which had long been used and esteemed as a mild and tasteless purgative; but the method of preparing it was not generally known before he made it public.[2]

It was originally obtained from a liquor called the _mother of nitre_, which is produced in the following manner:

Salt-petre is separated from the brine which first affords it, or from the water with which it is washed out of nitrous earths, by the process commonly used in crystallizing salts. In this process the brine is gradually diminished, and at length reduced to a small quantity of an unctuous bitter saline liquor, affording no more salt-petre by evaporation; but, if urged with a brisk fire, drying up into a confused mass which attracts water strongly, and becomes fluid again when exposed to the open air.

To this liquor the workmen have given the name of the _mother of nitre_; and _Hoffman_, finding it composed of the _magnesia_ united to an acid, obtained a separation of these, either by exposing the compound to a strong fire in which the acid was dissipated and the _magnesia_ remained behind, or by the addition of an alkali which attracted the acid to itself: and this last method he recommends as the best. He likewise makes an inquiry into the nature and virtues of the powder thus prepared; and observes, that it is an absorbent earth which joins readily with all acids, and must necessarily destroy any acidity it meets in the stomach; but that its purgative power is uncertain, for sometimes it has not the least effect of that kind. As it is a mere insipid earth, he rationally concludes it to be purgative only when converted into a sort of neutral salt by an acid in the stomach, and that its effect is therefore proportional to the quantity of this acid.

Altho' _magnesia_ appears from this history of it to be a very innocent medicine, yet having observed, that some hypochondriacs who used it frequently, were subject to flatulencies and spasms, he seems to have suspected it of some noxious quality. The circumstances however which gave rise to his suspicion, may very possibly have proceeded from the imprudence of his patients, who, trusting too much to _magnesia_, (which is properly a palliative in that disease,) and neglecting the assistance of other remedies, allowed their disorder to increase upon them. It may indeed be alledged, that _magnesia_, as a purgative, is not the most eligible medicine for such constitutions, as they agree best with those that strengthen, stimulate and warm; which the saline purges commonly used are not observed to do. But there seems at least to be no objection to its use when children are troubled with an acid in their stomach; for gentle purging in this case is very proper, and it is often more conveniently procured by means of _magnesia_ than of any other medicine, on account of its being intirely insipid.

The above-mentioned Author observing, some time after, that a bitter saline liquor, similar to that obtained from the brine of salt-petre, was likewise produced by the evaporation of those waters which contain common salt, had the curiosity to try if this would also yield a _magnesia_. The experiment succeeded: and he thus found out another process for obtaining this powder, and at the same time assured himself by experiments, that the product from both was exactly the same.[3]

My curiosity led me some time ago to inquire more particularly into the nature of _magnesia_, and especially to compare its properties with those of the other absorbent earths, of which there plainly appeared to me to be very different kinds, altho' commonly confounded together under one name. I was indeed led to this examination of the absorbent earths, partly by the hope of discovering a new sort of lime and lime-water, which might possibly be a more powerful solvent of the stone than that commonly used; but was disappointed in my expectations.

I have had no opportunity of seeing _Hoffman's_ first _magnesia_ or the liquor from which it is prepared, and have therefore been obliged to make my experiments upon the second.

In order to prepare it, I at first employed the bitter saline liquor called _bittern_, which remains in the pans after the evaporation of sea water. But as that liquor is not always easily procured, I afterwards made use of a salt called _epsom-salt_, which is separated from the bittern by crystallization, and is evidently composed of _magnesia_ and the vitriolic acid.

There is likewise a spurious kind of Glauber salt, which yields plenty of _magnesia_, and seems to be no other than the epsom salt of sea water reduced to crystals of a larger size. And common salt also affords a small quantity of this powder; because being separated from the bittern by one hasty crystallization only, it necessarily contains a portion of that liquor.

Those who would prepare a _magnesia_ from epsom-salt, may use the following process.

Dissolve equal quantities of epsom-salt, and of pearl ashes separately in a sufficient quantity of water; purify each solution from its dregs, and mix them accurately together by violent agitation: then make them just to boil over a brisk fire.

Add now to the mixture three or four times its quantity of hot water; after a little agitation, allow the _magnesia_ to settle to the bottom, and decant off as much of the water as possible. Pour on the same quantity of cold water; and, after settling, decant it off in the same manner. Repeat this washing with the cold water ten or twelve times: or even oftner, if the _magnesia_ be required perfectly pure for chemical experiments.

When it is sufficiently washed, the water may be strained and squeezed from it in a linen cloth; for very little of the _magnesia_ passes thro'.

The alkali in the mixture uniting with the acid, separates it from the _magnesia_; which not being of itself soluble in water, must consequently appear immediately under a solid form. But the powder which thus appears is not intirely _magnesia_; part of it is the neutral salt, formed from the union of the acid and alkali. This neutral salt is found, upon examination, to agree in all respects with vitriolated tartar, and requires a large quantity of hot water to dissolve it. As much of it is therefore dissolved as the water can take up; the rest is dispersed thro' the mixture in the form of a powder. Hence the necessity of washing the _magnesia_ with so much trouble; for the first affusion of hot water is intended to dissolve the whole of the salt, and the subsequent additions of cold water to wash away this solution.

The caution given of boiling the mixture is not unnecessary; if it be neglected, the whole of the _magnesia_ is not accurately separated at once; and by allowing it to rest for some time, that powder concretes into minute grains, which, when viewed with the microscope, appear to be assemblages of needles diverging from a point. This happens more especially when the solutions of the epsom-salt and of the alkali are diluted with too much water before they are mixed together. Thus, if a dram of epsom-salt and of salt of tartar be dissolved each in four ounces of water, and be mixed, and then allowed to rest three or four days, the whole of the _magnesia_ will be formed into these grains. Or if we filtrate the mixture soon after it is made, and heat the clear liquor which passes thro'; it will become turbid, and deposite a _magnesia_.

I had the curiosity to satisfy myself of the purgative power of _magnesia_, and of _Hoffman's_ opinion concerning it, by the following easy experiment. I made a neutral salt of _magnesia_ and distilled vinegar; choosing this acid as being, like that in weak stomachs, the product of fermentation. Six drams of this I dissolved in water, and gave to a middle-aged man, desiring him to take it by degrees. After having taken about a third, he desisted, and purged four times in an easy and gentle manner. A woman of a strong constitution got the remainder as a brisk purgative, and it operated ten times without causing any uneasiness. The taste of this salt is not disagreeable, and it appears to be rather of the cooling than of the acrid kind.

Having thus given a short sketch of the history and medical virtues of _magnesia_, I now proceed to an account of its chemical properties. By my first experiments, I intended to learn what sort of neutral salts might be obtained by joining it to each of the vulgar acids; and the result was as follows.

Magnesia is quickly dissolved with violent effervescence, or explosion of air, by the acids of vitriol, nitre, and of common salt, and by distilled vinegar; the neutral saline liquors thence produced having each their peculiar properties.

That which is made with the vitriolic acid, may be condensed into crystals similar in all respects to epsom-salt.

That which is made with the nitrous is of a yellow colour, and yields saline crystals, which retain their form in a very dry air, but melt in a moist one.

That which is produced by means of spirit of salt, yields no crystals; and if evaporated to dryness, soon melts again when exposed to the air.

That which is obtained from the union of distilled vinegar with _magnesia_, affords no crystals by evaporation, but is condensed into a saline mass, which, while warm, is extremely tough and viscid, very much resembling a strong glue both in colour and consistence, and becomes brittle when cold.

By these experiments _magnesia_ appears to be a substance very different from those of the calcarious class; under which I would be understood to comprehend all those that are converted into a perfect quick-lime in a strong fire, such as _lime-stone_, _marble_, _chalk_, those _spars_ and _marles_ which effervesce with aqua fortis, all _animal shells_ and the bodies called _lithophyta_. All of these, by being joined with acids, yield a set of compounds which are very different from those we have just now described. Thus, if a small quantity of any calcarious matter be reduced to a fine powder and thrown into spirit of vitriol, it is attacked by this acid with a brisk effervescence; but little or no dissolution ensues. It absorbs the acid, and remains united with it in the form of a white powder, at the bottom of the vessel, while the liquor has hardly any taste, and shews only a very light cloud upon the addition of alkali.[4]

The same white powder is also formed when spirit of vitriol is added to a calcarious earth dissolved in any other acid; the vitriolic expelling the other acid, and joining itself to the earth by a stronger attraction; and upon this account the _magnesia_ of sea-water seems to be different from either of those described by _Hoffman_. He says expressly, that the solutions of each of his powders, or, what is equivalent, that the liquors from which they are obtained, formed a coagulum, and deposited a white powder, when he added the vitriolic acid;[5] which experiment I have often tried with the marine bittern, but without success. The coagulum thus formed in the mother of nitre may be owing to a quantity of quick-lime contained in it; for quick-lime is used in extracting the salt-petre from its matrix. But it is more difficult to account for the difference between _Hoffman's_ bittern and ours, unless we will be satisfied to refer it to this, that he got his from the waters of salt springs, which may possibly be different from those of the sea.

Magnesia is not less remarkably distinguished from the calcarious earths, by joining it to the nitrous and vegetable acids, than to the vitriolic. Those earths, when combined with spirit of nitre, cannot be reduced to a crystalline form, and if they are dissolved in distilled vinegar, the mixture spontaneously dries up into a friable salt.

Having thus found _magnesia_ to differ from the common alkaline earths, the object of my next inquiry was its peculiar degree of attraction for acids, or what was the place due to it in Mr. _Geoffroy's_ table of elective attractions.

Three drams of _magnesia_ in fine powder, an ounce of salt ammoniac, and six ounces of water were mixed together, and digested six days in a retort joined to a receiver.

During the whole time, the neck of the retort was pointed a little upwards, and the most watery part of the vapour, which was condensed there, fell back into its body. In the beginning of the experiment, a volatile salt was therefore collected in a dry form in the receiver, and afterwards dissolved into spirit.

When all was cool, I found in the retort a saline liquor, some undissolved _magnesia_, and some salt ammoniac crystallized. The saline liquor was separated from the other two, and then mixed with the alkaline spirit. A coagulum was immediately formed, and a _magnesia_ precipitated from the mixture.

The _magnesia_ which had remained in the retort, when well washed and dried, weighed two scruples and fifteen grains.

We learn by the latter part of this experiment, that the attraction of the volatile alkali for acids is stronger than that of _magnesia_, since it separated this powder from the acid to which it was joined. But it also appears, that a gentle heat is capable of overcoming this superiority of attraction, and of gradually elevating the alkali, while it leaves the less volatile acid with the _magnesia_.

Dissolve a dram of any calcarious substance in the acid of nitre or of common salt, taking care that the solution be rendered perfectly neutral, or that no superfluous acid be added. Mix with this solution a dram of _magnesia_ in fine powder, and digest it in the heat of boiling water about twenty four hours; then dilute the mixture with double its quantity of water, and filtrate. The greatest part of the earth now left in the filtre is calcarious, and the liquor which passed thro', if mixed with a dissolved alkali, yields a white powder, the largest portion of which is a true _magnesia_.

From this experiment it appears, that an acid quits a calcarious earth to join itself to _magnesia_; but the exchange being performed slowly, some of the _magnesia_ is still undissolved, and part of the calcarious earth remains yet joined to the acid.

When a small quantity of _magnesia_ is thrown into a solution of the corrosive sublimate of mercury, it soon separates part of the mercury in the form of a dark red powder, and is itself dissolved.

Imagining that I perceived some resemblance between the properties of _magnesia_ and those of alkalis, I was led to try what change this substance would suffer from the addition of quick-lime, which alters in such a peculiar manner the alkaline salts.

Twenty seven grains of _magnesia_ in fine powder were mixed with eighteen ounces of lime-water in a flask, which was corked close and shaken frequently for four days. During this time, I frequently dipp'd into it little bits of paper, which were coloured with the juice of violets; and these became green as soon as they touched the water, until the fourth day, when their colour did not seem to be altered. The water being now poured off, was intirely insipid, and agreed in every chemical trial with pure water. The powder, after being perfectly well dried, weighed thirty seven grains. It did not dissolve intirely in spirit of vitriol; but, after a brisk effervescence, part of it subsided in the same manner as the calcarious earths, when mixed with this acid.

When I first tried this experiment, I was at the trouble of digesting the mixture in the heat of boiling water, and did not then know that it would succeed in the heat of the air. But Dr. _Alston_, who has obliged the world with many curious and useful discoveries on the subject of quick-lime, having had occasion to repeat it, I learned from him that heat is not necessary; and he has moreover added an useful purpose to which this property of _magnesia_ may be applied; I mean the sweetening of water at sea, with which lime may have been mixed to prevent its putrefaction.

That part of the dried powder which does not dissolve in spirit of vitriol, consists of the lime separated from the water.

Quick-lime itself is also rendered mild by _magnesia_, if these two are well rubbed together and infused with a small quantity of water.

By the following experiments, I proposed to know whether this substance could be reduced to a quick-lime.

An ounce of _magnesia_ was exposed in a crucible for about an hour to such a heat as is sufficient to melt copper. When taken out, it weighed three drams and one scruple, or had lost 7/12 of its former weight.

I repeated, with the _magnesia_ prepared in this manner, most of those experiments I had already made upon it before calcination, and the result was as follows.

It dissolves in all the acids, and with these composes salts exactly similar to those described in the first set of experiments: but what is particularly to be remarked, it is dissolved without any the least degree of effervescence.

It slowly precipitates the corrosive sublimate of mercury in the form of a black powder.

It separates the volatile alkali in salt ammoniac from the acid, when it is mixed with a warm solution of that salt. But it does not separate an acid from a calcarious earth, nor does it induce the least change upon lime-water.

Lastly, when a dram of it is digested with an ounce of water in a bottle for some hours, it does not make any the least change in the water. The _magnesia_, when dried, is found to have gained ten grains; but it neither effervesces with acids, nor does it sensibly affect lime-water.

Observing _magnesia_ to lose such a remarkable proportion of its weight in the fire, my next attempts were directed to the investigation of this volatile part, and, among other experiments, the following seemed to throw some light upon it.

Three ounces of _magnesia_ were distilled in a glass retort and receiver, the fire being gradually increased until the _magnesia_ was obscurely red hot. When all was cool, I found only five drams of a whitish water in the receiver, which had a faint smell of the spirit of hartshorn, gave a green colour to the juice of violets, and rendered the solutions of corrosive sublimate and of silver very slightly turbid. But it did not sensibly effervesce with acids.

The _magnesia_, when taken out of the retort, weighed an ounce, three drams, and thirty grains, or had lost more than the half of its weight. It still effervesced pretty briskly with acids, tho' not so strongly as before this operation.

The fire should have been raised here to the degree requisite for the perfect calcination of _magnesia_. But even from this imperfect experiment, it is evident, that of the volatile parts contained in that powder, a small proportion only is water; the rest cannot, it seems, be retained in vessels, under a visible form. Chemists have often observed, in their distillations, that part of a body has vanished from their senses, notwithstanding the utmost care to retain it; and they have always found, upon further inquiry, that subtile part to be air, which having been imprisoned in the body, under a solid form, was set free and rendered fluid and elastic by the fire. We may therefore safely conclude, that the volatile matter, lost in the calcination of _magnesia_, is mostly air; and hence the calcined _magnesia_ does not emit air, or make an effervescence, when mixed with acids.

The water, from its properties, seems to contain a small portion of volatile alkali, which was probably formed from the earth, air, and water, or from some of these combined together; and perhaps also from a small quantity of inflammable matter which adhered accidentally to the _magnesia_. Whenever Chemists meet with this salt, they are inclined to ascribe its origin to some animal, or putrid vegetable, substance; and this they have always done, when they obtained it from the calcarious earths, all of which afford a small quantity of it. There is, however, no doubt that it can sometimes be produced independently of any such mixture, since many fresh vegetables and tartar afford a considerable quantity of it. And how can it, in the present instance, be supposed, that any animal or vegetable matter adhered to the _magnesia_, while it was dissolved by an acid, separated from this by an alkali, and washed with so much water?

Two drams of _magnesia_ were calcined in a crucible, in the manner described above, and thus reduced to two scruples and twelve grains. This calcined _magnesia_ was dissolved in a sufficient quantity of spirit of vitriol, and then again separated from the acid by the addition of an alkali, of which a large quantity is necessary for this purpose. The _magnesia_ being very well washed and dryed, weighed one dram and fifty grains. It effervesced violently, or emitted a large quantity of air, when thrown into acids, formed a red powder when mixed with a solution of sublimate, separated the calcarious earths from an acid, and sweetened lime-water: and had thus recovered all those properties which it had but just now lost by calcination: nor had it only recovered its original properties, but acquired besides an addition of weight nearly equal to what had been lost in the fire; and, as it is found to effervesce with acids, part of the addition must certainly be air.

This air seems to have been furnished by the alkali from which it was separated by the acid; for Dr. _Hales_ has clearly proved, that alkaline salts contain a large quantity of fixed air, which they emit in great abundance when joined to a pure acid. In the present case, the alkali is really joined to an acid, but without any visible emission of air; and yet the air is not retained in it: for the neutral salt, into which it is converted, is the same in quantity, and in every other respect, as if the acid employed had not been previously saturated with _magnesia_, but offered to the alkali in its pure state, and had driven the air out of it in their conflict. It seems therefore evident, that the air was forced from the alkali by the acid, and lodged itself in the _magnesia_.

These considerations led me to try a few experiments, whereby I might know what quantity of air is expelled from an alkali, or from _magnesia_, by acids.

Two drams of a pure fixed alkaline salt, and an ounce of water, were put into a Florentine flask, which, together with its contents, weighed two ounces and two drams. Some oil of vitriol diluted with water was dropt in, until the salt was exactly saturated; which it was found to be, when two drams, two scruples, and three grains of this acid had been added. The vial with its contents now weighed two ounces, four drams, and fifteen grains. One scruple, therefore, and eight grains were lost during the ebullition, of which a trifling portion may be water, or something of the same kind. The rest is air.

The celebrated _Homberg_ has attempted to estimate the quantity of solid salt contained in a determined portion of the several acids. He saturated equal quantities of an alkali with each of them; and, observing the weight which the alkali had gained, after being perfectly dryed, took this for the quantity of solid salt contained in that share of the acid which performed the saturation. But we learn from the above experiment, that his estimate was not accurate, because the alkali loses weight as well as gains it.

Two drams of _magnesia_, treated exactly as the alkali in the last experiment, were just dissolved by four drams, one scruple, and seven grains of the same acid liquor, and lost one scruple and sixteen grains by the ebullition.

Two drams of _magnesia_ were reduced, by the action of a violent fire, to two scruples and twelve grains, with which the same process was repeated, as in the two last experiments; four drams, one scruple, and two grains of the same acid were required to compleat the solution, and no weight was lost in the experiment.

As in the separation of the volatile from the fixed parts of bodies, by means of heat, a small quantity of the latter is generally raised with the former; so the air and water, originally contained in the _magnesia_, and afterwards dissipated by the fire, seem to have carried off a small part of the fixed earth of this substance. This is probably the reason, why calcined _magnesia_ is saturated with a quantity of acid, somewhat less than what is required to dissolve it before calcination: and the same may be assigned as one cause which hinders us from restoring the whole of its original weight, by solution and precipitation.

I took care to dilute the vitriolic acid, in order to avoid the heat and ebullition which it would otherwise have excited in the water; and I chose a Florentine flask, on account of its lightness, capacity, and shape, which is peculiarly adapted to the experiment; for the vapours raised by the ebullition circulated for a short time, thro' the wide cavity of the vial, but were soon collected upon its sides, like dew, and none of them seemed to reach the neck, which continued perfectly dry to the end of the experiment.

We now perceive the reason, why crude and calcined _magnesia_, which differ in many respects from one another, agree however in composing the same kind of salt, when dissolved in any particular acid; for the crude _magnesia_ seems to differ from the calcined chiefly by containing a considerable quantity of air, which air is unavoidably dissipated and lost during the dissolution.

From our experiments, it seems probable, that the increase of weight which some metals acquire, by being first dissolved in acids, and then separated from them again by alkalis, proceeds from air furnished by the alkalis. And that in the _aurum fulminans_, which is prepared by the same means, this air adheres to the gold in such a peculiar manner, that, in a moderate degree of heat, the whole of it recovers its elasticity in the same instant of time; and thus, by the violent shock which it gives to the air around, produces the loud crack or fulmination of this powder. Those who will imagine the explosion of such a minute portion of fixed air, as can reside in the _aurum fulminans_, to be insufficient for the excessive loudness of the noise, will consider, that it is not a large quantity of motion communicated to the air, but rather a smart stroke which produces sound, and that the explosion of but a few particles of fixed air may be capable of causing a loud noise, provided they all recover their spring suddenly, and in the same instant.

The above experiments lead us also to conclude, that volatile alkalis, and the common absorbent earths, which lose their air by being joined to acids, but shew evident signs of their having recovered it, when separated from them by alkalis, received it from these alkalis which lost it in the instant of their joining with the acid.

The following are a few experiments upon three of the absorbent earths, made in order to compare them with one another, and with _magnesia_.

Suspecting that _magnesia_ might possibly be no other than a common calcarious earth, which had changed its nature, by having been previously combined with an acid, I saturated a small quantity of chalk with the muriatic acid, separated the acid from it again by means of a fixed alkali, and carefully washed away the whole of the salt.

The chalk when dryed was not found to have suffered any alteration; for it effervesced with the vitriolic acid, but did not dissolve in it; and when exposed to a violent fire, was converted into a quick-lime, in all respects similar to that obtained from common chalk.

In another experiment of the same kind, I used the vitriolic acid with the same event.

Any calcarious matter reduced to a fine powder, and thrown into a warm solution of alum, immediately raises a brisk effervescence. But the powder is not dissolved; it is rather increased in bulk: and if the addition be repeated until it is no longer accompanied with effervescence, the liquor loses all taste of the alum, and yields only a very light cloud upon the admixture of an alkali.

From this experiment we learn, that acids attract the calcarious earths more strongly than they do the earth of alum; and as the acid in this salt is exactly the same with the vitriolic, it composes with the calcarious earth a neutral substance, which is very difficultly soluble in water, and therefore falls down to the bottom of the vessel along with the earth of alum which is deprived of its acid. The light cloud formed by the alkali proceeds from the minute portion of the calcarious compound which saturates the water.

The earth of animal bones, when reduced to a fine powder and thrown into a diluted vitriolic acid, gradually absorbs the acid in the same manner as the calcarious earths, but without any remarkable effervescence. When it is added to the nitrous or to the muriatic acid, it is slowly dissolved. The compound liquor thence produced is extremely acrid, and still changes the colour of the juice of violets to a red, even after it is fully saturated with the absorbent. Distilled vinegar has little or no effect upon this earth; for after a long digestion it still retains its sour taste, and gives only a light cloud upon the addition of an alkali.

By dropping a dissolved fixed alkali into a warm solution of alum, I obtained the earth of this salt, which, after being well washed and dried, was found to have the following properties.

It is dissolved in every acid but very slowly, unless assisted by heat. The several solutions, when thoroughly saturated, are all astringent with a slight degree of an acid taste, and they also agree with a solution of alum in this, that they give a red colour to the infusion of turnsol.

Neither this earth, nor that of animal bones, can be converted into quick-lime by the strongest fire, nor do they suffer any change worth notice. Both of them seem to attract acids but weakly, and to alter their properties less when united to them than the other absorbents.