imitate the process which Hutton had imagined. It was possible, that a direct experiment might artificially combine great pressure with great heat, and that the result might be, that the senses would realize what the intellect had conceived.187 But the experiment had never been tried, and Hutton, who delighted in reasoning from ideas rather than from facts, was not likely to undertake it.188 He cast his speculation on the world, and left it to its fate.189 Fortunately, however, for the reception of his system, a very ingenious and skilful experimenter of that day, Sir James Hall, determined to test the speculation by an appeal to facts; and as nature did not supply the facts which he wanted, he created them for himself. He applied heat to powdered chalk, while, at the same time, with great delicacy of manipulation, he subjected the chalk to a pressure about equal to the weight of a column of water half a mile high. The result was, that, under that pressure, the volatile parts of the chalk were held together; the carbonic acid gas was unable to escape; the generation of quicklime was stopped; the ordinary operations of nature were baffled, and the whole composition, being preserved in its integrity, was fused, and, on

187 Hutton, however, did not believe that this could be done. "In the Theory of the Earth which was published, I was anxious to warn the reader against the notion that subterraneous heat and fusion could be compared with that which we induce by our chemical operations on mineral substances here upon the surface of the earth." Hutton's Theory of the Earth, vol. i. P. 251.

In

188 See, in the Life of Hutton, in Playfair's Works, vol. iv. p. 62 note, a curious remark on his indifference to experimental verification. numerable passages in his work indicate this tendency, and show his desire to reason immediately from general principles. Thus, in vol. i. p. 17, “Let us strictly examine our principles in order to avoid fallacy in our reasoning." "We are now, in reasoning from principles, come to a point decisive of the question." vol. i. p. 177. "Let us now reason from our principles." vol. ii. p. 308. Hence, his constantly expressed contempt for experience; as in vol. ii. p. 367, where he says that we must overcome those prejudices which contracted views of nature and magnified opinions of the experience of man may have begotten."

189 Playfair (Life of Hutton, p. 64) says that it drew "their attention" (i. e. the attention of "men of science"), 66 very slowly, so that several years elapsed before any one showed himself publicly concerned about it, either as an enemy or a friend." He adds, as one of the reasons of this, that it contained "too little detail of facts for a system which involved so much that was new, and opposite to the opinions generally received."

subsequently cooling, actually crystallized into solid marble.190 Never was triumph more complete. Never did a fact more fully confirm an idea.191 But, in the mind of Hutton, the idea preceded the fact by a long interval; since, before the fact was known, the theory had been raised, and the system which was built upon it had, indeed, been published several years. It, therefore, appears that one of the chief parts of the Huttonian Theory, and certainly its most successful part, was conceived in opposition to all preceding experience; that it pre-supposed a combination of events which no one had ever observed, and the mere possibility of which nothing but artificial experiment could prove; and, finally, that Hutton was so confident of the validity of his own method of inquiry, that he disdained to make the experiment himself, but left to another mind that empirical branch of the investigation which he deemed of little moment, but which we, in England, are taught to believe is the only safe foundation of physical research.192

190 The account of these experiments was read before the Royal Society of Edinburgh in 1805, and is printed in their Transactions, vol. vi. pp. 71185, Edinb. 1812, 4to. The general result was (pp. 148, 149), "That a pressure of 52 atmospheres, or 1700 feet of sea, is capable of forming a limestone in a proper heat; That under 86 atmospheres, answering nearly to 3000 feet, or about half a mile, a complete marble may be formed; and lastly, That, with a pressure of 173 atmospheres, or 5700 feet, that is little more than one mile of sea, the carbonate of lime is made to undergo complete fusion, and to act powerfully on other earths." See also p. 160: The carbonic acid of limestone cannot be constrained in heat by a pressure less than that of 1708 feet of sea." There is a short, and not very accurate, notice of these instructive experiments in Bakewell's Geology, London, 1838, pp. 249, 250.

66

191 As Sir James Hall says, "The truth of the most doubtful principle which Dr. Hutton has assumed, has thus been established by direct experiment." Transactions of the Royal Society of Edinburgh, vol. vi. p. 175.

192 See the remarks of Sir James Hall, in Transactions, vol. vi. pp. 74, 75. He observes that Hutton's "system, however, involves so many suppositions, apparently in contradiction to common experience, which meet us on the very threshold, that most men have hitherto been deterred from an investigation of its principles, and only a few individuals have justly appreciated its merits." "I conceived that the chemical effects ascribed by him to compression, ought, in the first place, to be investigated." "It occurred to me that this principle was susceptible of being established in a direct manner by experiment, and I urged him to make the attempt; but he always rejected this proposal, on account of the immensity of the natural agents, whose operation he supposed to lie far beyond the

I have now given an account of all the most important discoveries made by Scotland, in the eighteenth century, respecting the laws of the inorganic world. I have said nothing of Watt, because, although the steamengine, which we owe to him, is of incalculable importance, it is not a discovery, but an invention. An invention it may justly be termed, rather than an improvement.193 Notwithstanding what had been effected in the seventeenth century, by De Caus, Worcester, Papin, and Savery, and notwithstanding the later additions of Newcomen and others, the real orginality of Watt is unimpeachable. His engine was, essentially, a new invention; but, under its scientific aspect, it was merely a skilful adaptation of laws previously known; and one of its most important points, namely, the economy of heat, was a practical application of ideas promulgated by Black. 194 The only discovery made by Watt, was that of the composition of water. Though his claims are disputed by the friends of Cavendish, it would appear

reach of our imitation; and he seemed to imagine that any such attempt must undoubtedly fail, and thus throw discredit on opinions, already suficiently established, as he conceived, on other principles."

193 It may be traced back, certainly to the beginning of the seventeenth century, and probably still higher. Yet the popular opinion seems to be correct, that Watt was its real inventor; though, of course, he could not have done what he did, without his predecessors. This, however, may be said of all the most eminent and successful men, as well as of the most ordinary men.

194 On the obligations of Watt to Black, compare Brougham's Life of Watt (Brougham's Works, vol. i. pp. 25, 36-38, edit. Glasgow, 1855), with Muirhead's Life of Watt, second edit. London, 1859, pp. 66, 83. At p. 301, Mr. Muirhead says of Watt, that "his principal inventions connected with the steam-engine, with all their prodigious results, were founded, as we have seen, on the attentive observation of great philosophical truths; and the economy of fuel, increase of productive power, and saving of animal labour, which gradually ensued, all originated in the sagacious and careful thought with which he investigated the nature and properties of heat." But whatever investigations Watt made into heat, he discovered no new law respecting it, or, at all events, no new law which is large enough to be noted in the history of thermotics, considered purely as a science, and apart from practical application. Mr. Muirhead, in his interesting work which I have just quoted, has published (pp. 484-486) some remarks made on the subject by Watt, several years after the death of Black, which, though perfectly fair and candid, show that Watt had a rather confused notion of the real difference between an invention and a discovery.

that he was the first who ascertained that water, instead of being an element, is a compound of two gases. 195 This discovery was a considerable step in the history of chemical analysis, but it neither involved nor suggested any new law of nature, and has, therefore, no claim to mark an epoch in the history of the human mind.196 There is, however, one circumstance connected with it which is too characteristic to be passed over in silence. The discovery was made in 1783, by Watt, the Scotchman, and by Cavendish, the Englishman, neither of whom seems to have been aware of what the other was doing 197 But between the two there was this difference. Watt, for several years previously, had been speculating on the subject of water in connexion with air, and having, by Black's law of latent heat, associated them

195 Mr. Muirhead, in his Life of Watt, pp. 301-370, seems to have put the priority of Watt beyond further doubt; though he is somewhat hard upon Cavendish, who, there can be little question, made the discovery for himself.

196 I would not wish to diminish one jot of the veneration in which the great name of Watt is justly held. But when I find the opinion of Dr. Withering, the botanist, quoted, to the effect that his "abilities and acquirements placed him next, if not superior, to Newton" (Muirhead's Life of Watt, p. 302), I cannot but protest against such indiscriminate eulogy, which would rank Watt in the same class as one of those godlike intellects of which the whole world has not produced a score, and which are entitled to be termed inspired, if ever human being was so. Another instance of this injudicious panegyric, will be found in the same otherwise excellent work (Muirhead, pp. 324, 325), where we read that Watt's discovery that water consists of oxygen and hydrogen, was "the commencement of a new era, the dawn of a new day in physical inquiry, the real foundation of the new system of chemistry; nay, even a discovery 'perhaps of greater importance than any single fact which human ingenuity has ascertained either before or since." "

197 That there was no plagiarism on the part of Watt, we know from positive evidence; that there was none on the part of Cavendish, may be fairly presumed, both from the character of the man, and also from the fact that in the then state of chemical knowledge the discovery was imminent, and could not have been long delayed. It was antecedently probable that the composition of water would be ascertained by different persons at the same time, as we have seen in many other discoveries which have been simultaneously made, when the human mind, in that particular department of inquiry, had reached a certain point. We are too apt to suspect philosophers of stealing from each other, what their own abilities are sufficient to work out for themselves. It is, however, certain that Watt thought himself ill-treated by Cavendish. See Watt's Correspondence on the Compo sition of Water, London, 1846, pp. 48, 61.

together, he was prepared to believe that one is convertible into the other.198 The idea of an intimate analogy between the two bodies having once entered his mind, gradually ripened; and when he, at last, completed the discovery, it was merely by reasoning from data which others possessed besides himself. Instead of bringing to light new facts, he drew new conclusions from former ideas.199 Cavendish, on the other hand, obtained his result by the method natural to an Englishman. He did not venture to draw a fresh inference, until he had first ascertained some fresh facts. Indeed, his discovery was so completely an induction from his own experiments, that he omitted to take into consideration the theory of latent heat, from which Watt had reasoned, and

198 On 26th November 1783, he writes: "For many years I have entertained an opinion that air was a modification of water; which was originally founded on the facts, that in most cases where air was actually made, which should be distinguished from those wherein it is only extricated from substances containing it in their pores, or otherwise united to them in the state of air, the substances were such as were known to contain water as one of their constituent parts, yet no water was obtained in the processes, except what was known to be only loosely connected with them, such as the water of the crystallization of salts. This opinion arose from a discovery that the latent heat contained in steam diminished, in proportion as the sensible heat of the water from which it was produced, increased; or, in other words, that the latent heat of steam was less when it was produced under a greater pressure, or in a more dense state, and greater when it was produced under a less pressure, or in a less dense state; which led me to conclude, that when a very great degree of heat was necessary for the production of the steam, the latent heat would be wholly changed into sensible heat; and that, in such cases, the steam itself might suffer some remarkable change. I now abandon this opinion, in so far as relates to the change of water into air, as I think that may be accounted for on better principles." See this remarkable passage, which is quite decisive as to the real history of Watt's discovery, in Correspondence of James Watt on the Composition of Water, London, 1846, pp. 84, 85. Compare p. cxxiv. and p. 248 note.

199 In the paper which he communicated to the Royal Society, announc ing his discovery, he, well knowing the empirical character of the English mind, apologizes for this; and says, "I feel much reluctance to lay my thoughts on these subjects before the public in their present indigested state, and without having been able to bring them to the test of such experi ments as would confirm or refute them." Watt's Correspondence on the Dis covery of the Composition of Water, pp. 77, 78. Eleven months earlier, that is in December 1782, he writes (Ibid. p. 4): "Dr. Priestley has made a most surprising discovery, which seems to confirm my theory of water's undergoing some very remarkable change at the point where all its latent heat would be changed into sensible heat."