the propofition by the analytical procefs. This chain of inveftigation is evidently fo long, and of fuch flow formation, that we cannot, with any degree of correctnefs, appreciate the comparative merits of those who severally extended it; nor point out the particular link upon which the grand difcovery hangs. And the fame diftribution of praise is strictly proper in almost all the other inftances of fuccessful phyfical refearch. Even the compofition of light was only unfolded by degrees, and appears to have been wonderfully nearly difcovered by Grimaldi and others, with whose works Newton must have been acquainted. There are numerous proofs of fuch anticipations contained in the writings of Hooke and Mayow, as our chemical readers well know. Mr Robifon has, in his notes to these lectures, pointed out several new and remarkable inftances, more particularly from the works of the former.* The fame obfervations may be extended to the most important discoveries in abstract science. The method of fluxions itself may be traced through a long fucceffion of lefs elegant and lefs general inventions for finding quadratures and fubtangents.

But two great phyfical difcoveries feem to have followed this law of continuity, in fo flight a degree, that they may almost be allowed to form a cafe of exceptions to its operation. Thefe are, the univerfality of gravitation, and the combination of heat. As, before the time of Sir Ifaac Newton, the influence of weight was only known by the falling of heavy bodies, all ideas of the attraction of gravitation were connected with this particular line of its operation: the only language in which men had ever learnt to express themselves upon the fubject, was borrowed immediately from the particular cafe of vertical defcent, and confined to the very limited sphere of its occurrence. In like manner, before the time of Dr Black, all the knowledge we had of the matter, or the motion of heat, was intimately connected with the idea of a fubftance, or a ftate, by which the fenfe of touch is affected in a fpecific manner, and the dimenfions of bodies fenfibly increafed. The phlogifton of Stahl was evidently no inference from induction, even as modified and altered by his followers; neither was it the hypothefis of any peculiar qualities in the matter of heat: It was the affumption of a fubftance, different from every other with which we are acquainted, endued with qualities repugnant

to

Note 13. Vol. I. contains a very interefting account of Hooke's theory of combuftion. Its fimilarity to the antiphlogiftic doctrine is truly fingular; and Mr Kobifou is, fo far as we know, the firft writer who has remarked it. In p. 537, he fays that he only observed it in 1798; but there must be fome mistake in this; for he published a notice of it in his valuable article Pneumatics, § 37', Encyclopædia Bri tannica, which appeared in the year 1795.

to the univerfal properties of matter, and capable of producing every effect which the inventors might wish to explain. Phlogifton was indeed denominated the matter of heat and light; but it might as well have been called the reguline principle; and then, inftead of faying that the efcape of the matter of heat and light caufes the calcination of metals, the followers of Stahl would have faid, that the efcape of the reguline principle caufes the combuftion of inflammable bodies. It is evident, that no fpecific effect, no fubordination to the laws of chemical affinity, was ever afcribed to the fubftance which affects our fenfe with the feeling of heat, until Dr Black, from the most faithful and cautious examination of obvious facts, found, that this fubftance is capable of uniting with bodies, fo as not to affect our fenfes with the peculiar feeling of heat, and yet to produce upon thofe bodies the most important changes-in the fame manner that an acid, when combined with an alkali, ceafes to taste four, while it deftroys the acridity of the alkali, and forms a third body, poffeffing the noxious qualities of neither. This phyfical law, difcovered by the ftrictest induction, is applicable to the explanation of an infinite number of phæ omena: its operations actually occur in almost every chemical experiment, and its influence is perceived in all the great proceffes of nature. For a moft interefting detail of the fteps by which Dr Black was led to the knowledge of it, we refer our readers to the first volume of thefe lectures. This narrative appears to us a model of philofophical writing, as well as of induction; making allowance for the ftyle of converfation, which is obviously adopted as moft fuitable for a public elementary lec

ture.

The other great difcovery of Dr Black (the nature of the alka line earths, and of fixed air) was fcarcely lefs important in its confequences to chemical fcience, than the one which we have been Lontemplating. The account of it contained in these volumes, differs confiderably from that which the author himself published. It dwells more minutely upon the fteps of the inveftigation, and (with the exception of a few remarks upon borax, apparently copied from the treatife on magnefia alba and quicklime), it is compofed in a style much less careful and finished than that which the Doctor feems to have employed when he wrote for publication.

Our readers will derive fome entertainment from the comparifon; and we shall here give, as a fpecimen of the manner which diftinguishes the whole of thefe lectures, the following paffage from the history of the difcovery of fixed air. It unites, with great fimplicity of diction, an exemplification of Dr Black's inimitable fimplicity and ingenuity in the contrivance of experi

ments.

• In

In the fame year in which my first account of these experiments was published, namely 1757, I had discovered, that this particular kind of air, attracted by alkaline fubftances, is deadly to all animals that breathe it by the mouth and noftrils together; but that if the noftrils were kept hut, I was led to think that it might be breathed with fafety. 1 found, for example, that when fparrows died in it in ten or eleven feconds, they would live in it for three or four minutes when the noftrils were fhut by melted fuet. And I convinced myself, that the change produced on wholefome air by breathing it, confifted chiefly, if not folely, in the converfion of part of it into fixed air. For I found, that by blowing through a pipe into lime water, or a folution of cauftic alkali, the lime was precipitated, and the alkali was rendered mild. I was partly led to thefe experiments by fome obfervations of Dr Hales, in which he fays, that breathing through diaphragms of cloth dipped in alkaline folution, made the air laft longer for the purposes of life.*

In the fame year, I found that fixed air is the chief part of the elaftic matter which is formed in liquids in the vinous fermentation. Van Helmont had indeed faid this, and it was to this that he first gave the name gas filveftre. It could not long be unknown to those occupied in brewing or making wines. But it was at random that he said it was the fame with that of the Grotto del Cane in Italy (but he fuppofed the identity, because both are deadly); for he had examined neither of them chemically, nor did he know that it was the air difengaged in the effervefcence of alkaline fubftances with acids. I convinced myself of the fact, by going to a brew-house with two phials, one filled with diftilled water, and the other with lime-water. 1 emptied the first into a vat of wort fermenting briskly, holding the mouth of the phial clofe to the furface of the wort. I then poured fome of the lime-water into it, fhut it with my finger, and shook it. The lime-water became turbid immediately.

Van Helmont fays, that the dunfle, or deadly vapour of burning charcoal, is the fame gas filveftre; but this was also a random conjecture. He does not even say that it extinguifhes flame; yet this was known to the chemifts of his day. I had now the certain means of deciding the queftion, fince, if the fame, it must be fixed air. I made feveral indiftinct experiments, as foon as the conjecture occurred to my thoughts; but they were with little contrivance or accuracy. In the evening of the fame day that I difcovered that it was fixed air that efcaped from fermenting liquors, I made an experiment which fatisfied me. Unfixing the nozzle of a pair of chamber bellows, I put a bit of charcoal, juft red hot, into the wide end of it, and then quickly putting it into its place again, I plunged the pipe to the bottom of a

phial,

*In the winter 1764-5, Dr Black rendered a confiderable quantity of cauftic foffil alkali mild and cryftalline, by caufing it to filtre flowly by rage, in an apparatus which was placed above one of the fpiracles in the ceiling of a church, in which a congregation of more than 1500 perfons had continued near ten hours.-Editor.

phial, and forced the air very flowly through the charcoal, fo as to maintain its combuftion, but not produce a heat too fuddenly for the phial to bear. When I judged that the air of the phial was completely vitiated, I poured lime-water into it, and had the pleasure of feeing it become milky in a moment.' Vol. II. p. 87. 88.

We cannot easily imagine a more interefting narrative; it reminds us of Montucla's admirable and animating account of the Torricellian experiment, with this effential difference, that here the narrator was himself the performer of the action.

The following paffage, from the concluding difcourfe on heat, may ferve as an example of Dr Black's powers of defcription; and we cannot help regretting, that thefe volumes do not contain also the lecture in which he was wont (unneceffarily, indeed, but with great force of invective) to expofe the manifold abfurdities of Meyer's acidum pingue.

It is plain, that not only all animal and vegetable life, but that the whole face and appearance of nature, the very form and powers of the elements themselves, depend on this limited action of heat. There are none of the elementary bodies with which we are better acquainted than water. Let us attend a little to the powers and qualities by which it acts its part in this fyftem of beings. We all admire its pure tranfparency in a fpring; the level and polished furface with which it reflects objects that are on the banks of a lake; the mobility with which it runs along the channel of a brook, and the inceffant motion of its waves in a ftormy fea. But, when viewed with a philofophical eye, it ap pears much more an object of admiration. The fame water which, under its ufual form, is fuch a principal beauty in the scene of nature, is employed in her moft extenfive operations, and is neceffary to the formation of all her productions. It penetrates the interior parts of the earth, and appears to affift in the production of various minerals, ftones, and earths, found there, by bringing their different ingredients together, and applying them to one another properly, that they may concrete. We know it arifes in vapours from the furface of the ocean, to form the clouds, and to defcend again in rain upon the dry land, and give origin to fprings, rivers, and lakes; or, upon proper occafions, to form deep fnow, which protects the ground and vegetables from the intense and mortal cold to which fome parts of the world are expofed; and, after it has performed this ufeful office, it readily yields to the heat of fummer, and returns to a ftate in which it ferves the fame purpofes as rain. By its fluidity and tenuity, it penetrates the foil, and the feeds of plants which that foil contains. Thefe it caufes to fwell and germinate into plants, which depend on water for fupport. It paffes with freedom and cafe through all their minutest tubes and veffels, and carries with it materials neceffary for nourishment and growth, or changes its appearance fo as to become part of the plant... There is no plant or vegetable fubftance, that does not contain in its compofition a large quantity of water, eafily feparable from it. The hardest woods

contain

contain à great deal. The fofter and more fucculent parts of vegetables are almoft totally composed of it. Even the oils and refinous substances can be refolved in part into water. It is plainly as necessary to the animals, and is found to be as copious an ingredient in the compofition of their bodies, and of all the different parts of them,

Thefe are the numerous and extenfive ufes of this beautiful element. But, in this fucceffion of forms and operations which it undergoes, you will perceive that it is fet in motion, and adapted to these ends, by the nice adjustment and gentle viciffitudes of heat and cold, which attend the returns of day and night, and fummer and winter ; and that even the form, under which it and the other clements play their parts, depends on the limited action of heat. Were our heat to be diminished, and to continue diminished, to a degree not very far below the ordinary temperature, the water would lofe its fluidity, and affume the form of a folid hard body, totally unfit for the numerous purposes which it ferves at prefent. And, if the diminution of heat were to go ftill farther, the air itself would lose its elafticity, and would be frozen to a folid useless matter like the water; and thus all nature would become a lifeless, filent, and difmal ruin. Such being the important part allotted to water, in the magnificent feries of natural operations, in confequence of the qualities communicated to it by heat, all its properties become interefting objects of contemplation to a fenfible heart. That peculiarity, by which the expanfion and contraction of water by heat is diftinguifhed from the fame effect on other fubftances, 1 mean its irregularity between 32° and 40° of Fahrenheit, naturally attracts attention. Even this feemingly trifling diftinétion has been fhewn by Count Rumford to have a mighty effect in rendering our habitation more comfortable.

On the other hand, were the heat which at prefent cherishes and enlivens this globe, allowed to increase beyond the bounds at prefent prefcribed to it; befide the deftruction of all animal and vegetable life, which would be the immediate and inevitable confequence, the water would lofe its prefent form, and affume that of an elastic vapour like air; the folid parts of the globe would be melted and confounded together, or mixed with the air and water in smoke and vapour; and nature would return to the original chaos. Vol. I. p. 245-247.

Of the converfation ftyle, in which thefe lectures are for the most part written, we may remark, that although it ufually poffeffes the advantages of plainnefs and fluency, yet, being adapted to the tones of the voice, it is very apt, when read over by a third perfon, to be deficient in perfpicuity; and being lefs premeditated, it is fcarcely ever equal in precifion to a good written. ftyle. Its want of elegance is a defect of much lefs confequence; but all these circumftances muft confpire to impair the effect of this work, unless the occafion of its compofition be kept in view. As a fyftem of chemical inftruction, the lectures of Dr Black poffefs very peculiar merits. Although they are, in many im, portant refpects, of neceffity far behind the more recent fyftema