作者：Joseph Black
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Experiments upon magnesia alba, Quicklime, and some other Alcaline Substances试读：
PREFACE.
lack's Paper entitled "Experiments upon Magnesia Alba, Quicklime, and some other Alcaline Substances" was read in BJune 1755, and was first published in "Essays and Observations, Physical and Literary．Read before a Society in Edinburgh, and Published by them," Volume II., Edinburgh, 1756; pp.157-225．It was subsequently reprinted several times during the life of the author, not only in later editions of these Essays, but also in a separate form．Copies of the original Paper are now very difficult to obtain, and the later reprints have also become scarce.The present reprint is a faithful copy of the Paper as it first appeared in 1756, the spelling, ＆c., of the original having been carefully reproduced.
The Paper constitutes a highly important step in the laying of the foundations of chemistry as an exact science, and furnishes a model of carefully planned experimental investigation, and of clear reasoning upon the results of experiment．It is neither so widely read by the younger chemists nor is it so readily accessible as it ought to be, and the object of the Alembic Club in issuing it as the first volume of a series of Reprints of historically important contributions to Chemistry, is to place it within easy reach of every student of Chemistry and of the History of Chemistry.
The student's attention may be particularly called to Black's tacit adoption of the quantitative method in a large number of his experiments, and to the way in which he bases many of his conclusions upon the results obtained in these experiments．Even yet it is very frequently stated that the introduction of the quantitative method into Chemistry (which did not by any means originate with Black) took place at a considerably later date.L．D.
EXPERIMENTS UPON MAGNESIA ALBA, QUICKLIME, AND SOME OTHER ALCALINE SUBSTANCES; By JOSEPH BLACK, M.D.
PART I.
offman , in one of his observations, gives the history of a powder calledmagnesia alba, which had long been used and esteemed Has a mild and tasteless purgative; but the method of preparing it was not generally known before he made it public.It was originally obtained from a liquor called themother 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 themother of nitre; andHoffman, finding it composed of themagnesiaunited to an acid, obtained a separation of these, either by exposing the compound to a strong fire in which the acid was dissipated and themagnesiaremained 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'magnesiaappears 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 tomagnesia, (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, thatmagnesia, 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 ofmagnesiathan 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 amagnesia．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.
My curiosity led me some time ago to inquire more particularly into the nature ofmagnesia, 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 seeingHoffman'sfirstmagnesiaor 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 calledbittern, 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 calledepsom-salt, which is separated from the bittern by crystallization, and is evidently composed ofmagnesiaand the vitriolic acid.
There is likewise a spurious kind of Glauber salt, which yields plenty ofmagnesia, 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 amagnesiafrom 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 themagnesiato 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 themagnesiabe 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 themagnesiapasses thro'.
The alkali in the mixture uniting with the acid, separates it from themagnesia; which not being of itself soluble in water, must consequently appear immediately under a solid form．But the powder which thus appears is not intirelymagnesia; 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 themagnesiawith 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 themagnesiais 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 themagnesiawill 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 amagnesia.
I had the curiosity to satisfy myself of the purgative power ofmagnesia, and ofHoffman'sopinion concerning it, by the following easy experiment．I made a neutral salt ofmagnesiaand 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 ofmagnesia, 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 withmagnesia, 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 experimentsmagnesiaappears 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 aslime-stone,marble,chalk, thosesparsandmarleswhich effervesce with aqua fortis, allanimal shellsand the bodies calledlithophyta．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.
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 themagnesiaof sea-water seems to be different from either of those described byHoffman．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;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 betweenHoffman'sbittern 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 foundmagnesiato 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'stable of elective attractions.
Three drams ofmagnesiain 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 undissolvedmagnesia, 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 amagnesiaprecipitated from the mixture.
Themagnesiawhich 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 ofmagnesia, 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 themagnesia.
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 ofmagnesiain 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 truemagnesia.
From this experiment it appears, that an acid quits a calcarious earth to join itself tomagnesia; but the exchange being performed slowly, some of themagnesiais still undissolved, and part of the calcarious earth remains yet joined to the acid.
When a small quantity ofmagnesiais 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 ofmagnesiaand 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 ofmagnesiain 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 ofmagnesiamay 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 bymagnesia, 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 ofmagnesiawas 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 themagnesiaprepared 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．Themagnesia, when dried, is found to have gained ten grains; but it neither effervesces with acids, nor does it sensibly affect lime-water.
Observingmagnesiato 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 ofmagnesiawere distilled in a glass retort and receiver, the fire being gradually increased until themagnesiawas 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.
Themagnesia, 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 ofmagnesia．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 ofmagnesia, is mostly air; and hence the calcinedmagnesiadoes not
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