Experiments upon Magnesia Alba, Quick-Lime, and other Alcaline Substances

"Hoffman, in one of his observations, gives the history of a powder called Magnesia Alba, which had been long used, and esteemed as 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 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 saltpetre 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 a 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."

"[Hoffman] 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."

"I have... been obliged to make my experiments upon the second [method]. ...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."

"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 dipped 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 entirely 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 entirely in spirit of vitriol [dilute sulfuric acid]; but, after a brisk effervescence, part of it subsided, in the same manner as the calcareous earths, when mixed with this acid."

"Dr Alston... has... added an useful purpose, to which this property of magnesia may be applied; I mean the sweetening of water [in sailing ships] 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... is... 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."

"[M]agnesia was exposed in a crucible, for about an hour, to such a heat as is sufficient to melt copper. When taken out, it weighed 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... it is dissolved without any the least degree of effervescence."

"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..."

"[I]t 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 of magnesia, is mostly air; and hence the calcined magnesia does not emit air, or make an effervescence when mixed with acids."

"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."

"It is sufficiently clear, that the earths in their native state, and that the s and magnesia in their ordinary condition, contain a large quantity of fixed air; and this air certainly adheres to them with considerable force, since a strong fire is necessary to separate it from magnesia; and the strongest is not sufficient to expel it entirely from fixed alkalis, or take away their power of effervescing with acid salts."

"These considerations led me to conclude, that the relation between fixed air and ne substances, was somewhat similar to the relation between these and s: that as the earths and alkalis attract acids strongly, and can be saturated with them, so they also attract fixed air, and are, in their ordinary state, saturated with it: and, when we mix an acid with an alkali, or with an absorbent earth, that the air is then set at liberty, and breaks out with violence; because the alkaline body attracts it more weakly than it does the acid, and because the acid and air cannot both be joined to the same body at the same time."

"I also imagined, that when the earths are exposed to the action of a violent fire, and are thereby converted into quick-lime, they suffer no other change in their composition, than the loss of a small quantity of water, and of their fixed air. The remarkable acrimony which we perceive in them after this process, was not supposed to proceed from any additional matter received the fire, but seemed to be an essential property of the pure earth, depending upon an attraction for those several substances which it then became capable of corroding or dissolving; which attraction had been insensible as long as the air adhered to the earth, but discovered itself upon the separation."

"This supposition was founded upon an observation of the most frequent consequences of combining bodies in chemistry."

"Commonly, when we join two bodies together, their acrimony or attraction for other substances becomes immediately either less perceivable, or entirely insensible; although it was sufficiently strong and remarkable before their union, and may be rendered evident again by disjoining them."

"A neutral salt, which is composed of an acid and alkali, does not possess the acrimony of either of its constituent parts. It can easily be separated from water, has little or no effect upon metals, is incapable of being joined to inflammable bodies, and of corroding and dissolving animals and vegetables; so that the attraction both of the acid and alkali for these several substances, seems to be suspended till they are again separated from one another."

"Crude lime was therefore considered as a peculiar acrid earth, rendered mild by its union with fixed air; and quick-lime as the same earth, in which, by having separated the air, we discover that acrimony or attraction for water, for animal, vegetable, and for inflammable substances."

"That the calcareous earths really lose a large quantity of air when they are burnt to quick-lime, seems sufficiently proved by an experiment of Mr Margraaf, an exceedingly accurate and judicious chemist."

"A earth, deprived of its [fixed] air, or in the state of quick-lime, greedily absorbs a considerable quantity of water, becomes soluble in that fluid, and is then said to be slaked; but as soon as it meets with fixed air, it is supposed to quit the water, and join itself to the air, for which it has a superior attraction, and is therefore restored to its first state of mildness and insolubility in water."

"When slaked lime is mixed with water, the fixed air [dissolved] in the water is attracted by the lime, and saturates a small portion of it, which then becomes again incapable of dissolution, but part of the remaining slaked lime is dissolved and composes lime-water."

"If this fluid be exposed to the open air, the particles of quick-lime which are nearest the surface, gradually attract the particles of fixed air which float in the atmosphere. But at the same time that a particle of lime is thus saturated with air, it is also restored to its native state of mildness and insolubility; and as the whole of this change must happen at the surface, the whole of the lime is successively collected there, under its original form of an insipid calcarious earth, called the cream or crusts of lime-water."

"When quick-lime itself is exposed to the open air, it absorbs the particles of water, and of fixed air, which come within its sphere of attraction. As it meets with the first of these in greatest plenty, the greatest part of it assumes the form of slaked lime; the rest is restored to its original state: and if it be exposed for a sufficient length of time, the whole of it is gradually saturated with air, to which the water as gradually yields its place."

"Black's Paper entitled "Experiments upon Magnesia Alba, Quicklime, and some other Alcaline Substances" was read in June 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."

"Black... is best remembered by chemists for his work on magnesia alba which he presented for the doctor's degree in 1754. In this investigation he took up the study of what we now call as a new substance which he desired to characterize, and he proceeded to try some experiments upon it from which he was able to draw important conclusions. ...[W]e may sum up Black's results in a series of propositions: I. Magnesia alba when strongly heated loses about half its weight and yields a new substance magnesia usta () II. With vitriolic acid magnesia alba yields epsom salt () with effervescence. III. Magnesia usta when similarly treated yields epsom salt without effervescence. IV. In a solution of epsom salt, mild alkali () precipitates magnesia alba and the solution on evaporation yields vitriolated tartar (). V. Mild alkali effervesces with acids while caustic does not. VI. Mild alkali is made caustic by addition of magnesia usta."

"[I]t is particularly easy to see what a handicap upon the chemists of that time was the use of a nomenclature necessarily incapable of expressing chemical relationships, and how impossible it then was to know whether all substances in a reaction were accounted for or not. Nevertheless, Black interpreted his results with perfect accuracy."

"From II and III he concluded that the difference between magnesia alba and magnesia usta was the gas ("fixed air") liberated from the former by s, and that it was the expulsion of the same gas which accounted for the loss of weight when magnesia alba was heated (I)."

"II and IV showed that magnesia alba could be regenerated from magnesia usta by the aid of mild alkali, hence the latter must contain fixed air which it surrenders in the reaction. This is further confirmed by V which shows that mild alkali differs from caustic by its content of fixed air."

"Finally VI completes the caustifying of the alkali by the action of magnesia [usta]."

"Black saw at once that these reactions were analogous to those involved in the ancient method of preparing caustic alkali from quicklime. He accordingly repeated his experiments using instead of magnesia alba and so reached the correct conclusion that the 'burning' of lime consists essentially in the expulsion of fixed air."

"Such a result was utterly opposed to the explanations hitherto current. According to the latter, when lime was heated in the kiln phlogiston entered into it making it fiery or caustic. Later when the quicklime was treated with mild alkali another transfer of phlogiston occurred and the latter became caustic in its turn. It is what might have been expected from his clear habits of thought that in later years, when Lavoisier had once shown the way, Black was among the first to adopt the new views."

"Black was Professor of Chemistry at Glasgow and Edinburgh successively, and although he published only three papers on chemical subjects, these were models of accuracy and logic, and may still be read with profit by the novice—and indeed by the mature chemist. The most important of the three is entitled Experiments upon Magnesia Alba, Quicklime, and some other Alcaline Substances, published in 1756. A modern reprint of it was made by the Alembic Club..."

"Black set out to solve... the nature of the changes which occur when quicklime is added to the 'mild' alkalis (potassium and s) to render them caustic, that is... to convert them into potassium and s."

"The caustic nature of the formed when is strongly heated was explained on the phlogiston theory by assuming that the chalk had taken up phlogiston from the fire. Black, however, observed (1) that a loss in weight occurs when chalk is converted into quicklime, and (2) that this loss in weight is due to the fact that a gas, fixed air [], is evolved in the reaction. He found, moreover, that magnesia alba... underwent a similar change when strongly heated, but that heat had no effect upon the fixed 'mild' alkalis. ...and ...that two drams of chalk lost the same weight of air when treated with a dilute acid as when heated strongly in a furnace."

"He next showed that if a definite weight of was taken and converted into quicklime, the latter could be reconverted into chalk by treatment with a solution of a mild alkali, and that the weight of the chalk thus formed was equal to that of the original specimen. The lime therefore had been "saturated with [fixed] air which must have been furnished by the alkali"."

"On exposing a solution of caustic alkali to the air for some time, Black found that 'the alkali lost the whole of its causticity, and seemed entirely restored to the state of an ordinary fixed alkali', and this he explained by assuming that the caustic alkali had absorbed fixed air from the atmosphere."

"From this remarkable series of experiments he had thus obtained results which enabled him to explain satisfactorily the whole problem. was a compound of quicklime with fixed air; when heated it lost the fixed air, quicklime being left. The mild alkalis were compounds of fixed air with substances resembling quicklime, but much more soluble in water."

"When a solution of a mild alkali was treated with quicklime, the latter absorbed the fixed air of the former, with production of insoluble or ; the filtrate was therefore a solution of caustic alkali."