452 



NATURE 



[September 4, 1890 



greater safety. The results of these observations are contained 

 in Priestley's " Experiments and Observations on Air," vol. v. 

 1 781, App., p. 395- 



At this period Cavendish was engaged on a series of experi- 

 ments '• made, as he says, principally with a view to find out 

 the cause of the diminution which common air is well known to 

 suffer by all the various ways in which it is phlogisticated, and 

 to discover what becomes of the air thus lost or condensed " 

 (Cavendish, Phil. Trans. 1784, p. 119). On the publication 

 of Priestley's work he repeated Waltire's experiment, for, he 

 says, as it " seemed likely to throw great light on the subject I 

 had in view, I thought it well worth examining more closely." 

 The series of experiments which Cavendish was thus induced to 

 make, and which he made with all his wonted skill in quantita- 

 tive work, led him some time in the summer of 1781 to the 

 discovery that a mixture of two volumes of the inflammable air 

 from metals (the gas we now call hydrogen) with one volume of 

 the dephlogisticated air of Priestley combine together under the 

 influence of the electric spark, or by burning, to form the same 

 weight of water. If Cavendish had published the results of 

 these observations at or near the time he obtained them, there 

 would have been no Water Controversy. But in the course of 

 the trials he found that the condensed water was sometimes acid, 

 and the search for the cause of the acidity (which incidentally 

 ltd to the discovery of the composition of nitric acid) occasioned 

 the delay. The main result that a mixture of two volumes of 

 inflammable air and one volume of dephlogisticated air could be 

 converted into the same weight of water was, however, com- 

 municated to Priestley, as he relates in a paper in the Phil. 

 Trans, for 1783. Priestley was at this time interested in an 

 investigation on the seeming convertibility of water into air, and 

 he was led to repeat Cavendish's experiments, some time in 

 March 1783, on what was apparently the converse problem. 

 Priestley, however, made a fatal blunder in the repetition. 

 With the praiseworthy idea of obviating the possibility of 

 any moisture in the gases, he prepared the dephlogisticated 

 air from nitre, and the inflammable air by heating what he 

 calls "perfectly made charcoal" in an earthenware retort. 

 At this time, it must be remembered, there was no sharp dis- 

 tinction between the various kinds of inflammable air : hydrogen, 

 sulphuretted hydrogen, marsh gas and olefiant gas, coal gas, the 

 vapours of ether and turpentine, and the gas from heated char- 

 coal, consisting of a mixture of carbonic oxide, marsh gas, and 

 carbonic acid, were indifferently termed "inflammable air." 

 Priestley attempted to verify Cavendish's conclusion on the 

 identity of the weight of the gases used with that of the water 

 formed ; but his method in this respect, as in his choice of the 

 inflammable air, was wholly defective, and could not possibly 

 have given him accurate results. It consisted in wiping out the 

 water from the explosion vessel by means of a weighed piece of 

 blotting-paper and determining the increase of weight of the 

 paper. He says, however : — "I always found as near as I could 

 judge the weight of the decomposed air in the moisture acquired 

 by the paper. ... I wished, however, to have had a nicer 

 balance for this purpose ; the result was such as to afford a strong 

 presumption that the air was reconverted into water, and there- 

 iore that the origin of it had been water." These results, 

 together with those on the conversion of water into air, were 

 communicated towards the end of March 1783 by Priestley to 

 Watt, who began to theorize upon them, and then to put his 

 thoughts together in the form of a letter to Priestley, dated April 

 26, 1783, and which he requested might be read to the Royal 

 Society on the occasion of the presentation of Priestley's memoir. 

 In this letter Watt says : — "Let us now consider what obviously 

 happens in the case of the deflagration of the inflammable and 

 dephlogisticated air. These two kinds of air unite with violence, 

 they become red-hot, and upon cooling totally disappear. When 

 the vessel is cooled, a quantity of water is found in it equal to 

 the weight of the air employed. This water is then the only re- 

 maining product of the process, and water, light, and heat are 

 all the products. Are we not then authorized to conclude that 

 water is composed of dephlogisticated air and phlogiston deprived 

 of part of their latent or elementary heat ; that dephlogisticated or 

 pure air is composed of water deprived of its phlogiston and 

 taiited to elementary heat and light, ^c. ? " 



This letter, although shown to several Fellows of the Society, 

 V as not publicly read at the time intended. Priestley, before its 

 receipt, had detected the fallacy of his experiments on the 

 seeming conversion of water into air, and as much of the letter 

 was concerned with this matter Watt requested that it should be 



NO. I 



, VOL. 42] 



withdrawn. Watt, however, as he tells Black 1 in a letter dated 

 June 23, 1783, had not given up his theory as to the nature of 

 water, and on November 26, 1783, he restated his views more 

 fully in a letter to De Luc. In the meantime. Cavendish, having 

 completed one section of his investigation, sent in a memoir to 

 the Royal Society, which was read on January 15, 1784, in 

 which he gives an account of his experiments, and announces his 

 conclusion "that dephlogisticated air is in reality nothing but de- 

 phlogisticated water, or water deprived of its phlogiston ; or, in 

 other words, that water consists of dephlogisticated air united to 

 phlogiston ; and that inflammable air is either pure phlogiston, 

 as Dr. Priestley and Mr. Kirwan suppose, or else water united 

 to phlogiston." Watt thereupon requested that his letter to De 

 Luc should be published, and it was accordingly read to the 

 Royal Society on April 29, 1784. Which of the two— Caven- 

 dish or Watt — is, under these circumstances, to be considered as 

 " the true and first discoverer " of the compound nature of water 

 is the question which has been hitherto the main subject of the 

 Water Controversy. 



Let us now consider the matter as it affects Lavoisier. In 

 1783, Lavoisier had publicly declared against the doctrine of 

 phlogiston, or rather, as M. Dumas puts it, " against the crowd 

 of entities of that name which had no quality in common except 

 that of being intangible by every known method" (" Lefons 

 sur la Philosophic Chimique," p. 161). How completely 

 Lavoisier had dissociated himself from the theory may be 

 gleaned from his memoir of that year. " Chemists," he says, 

 "have made a vague principle of phlogiston which is not strictly 

 defined, and which in consequence accommodates itself to every 

 explanation into which it is pressed. Sometimes this principU- 

 is heavy and sometimes it is not ; sometimes it is free fire and 

 sometimes it is fire combined with the earthly element ; some- 

 times it passes through the pores of vessels and sometimes the>' 

 are impenetrable to it : it explains at once causticity and non- 

 causticity, transparency and opacity, colours and the absence 

 of colours. It is a veritable Proteus which changes its form every 

 moment." 



But Lavoisier had merely renounced one fetich for another. 

 At the time that he penned these lines he was as much under 

 the thraldom oi le principe oxygine as the most devoted follower 

 of Stahl was in the bondage of phlogiston. The idea that the 

 calcination of metals was but a slow combustion had been fully 

 recognized. M. Berthelot tells us that, as far back as the March 

 of 1774, Lavoisier had written in his laboratory journal : — "I am 

 persuaded that the inflammation of inflammable air is nothing 

 but a fixation of a portion of the atmospheric air, a decomposi- 

 tion of air. ... In that case in every inflammation of air there 

 ought to be an increase of weight, and he tried to ascertain this 

 by burning hydrogen at the mouth of a vessel from which it was 

 being disengaged. In the following year he asks. What remains 

 when inflammable air is burnt completely? According to the 

 theory by which he is now swayed it should be an acid, and h(* 

 made many attempts to capture this acid. In 1777 he and Bucquet 

 burnt six pints of the inflammable air from metals in a bottlr 

 containing lime-water, in the expectation that fixed air would be 

 the result. And in 1781 he repeated the experiment with 

 Gengembre, with the modification that the oxygen was caused 

 to burn in an atmosphere of hydrogen, but not a trace of any 

 acid product could be detected. Of course there must have been 

 considerable quantities of water formed in these experiments, 

 but Lavoisier was preoccupied with the conviction that oxida- 

 tion meant acidification, and its presence was unnoticed, or, if 

 noticed, was unheeded. Macquer, in 1776, had drawn attention 

 to the formation of water during the combustion of hydrogen in 

 air, but Lavoisier has stated that he was ignorant of that ob- 

 servation. What was it then that put him on the right track ? 

 We venture to think that M. Berthelot has himself supplied the 

 answer. He says (p. 114): — " Rumours of Cavendish's trials 

 had spread throughout the scientific world during the spring of 

 1783. . . . Lavoisier, always on the alert as to the nature of 

 the products of the combustion of hydrogen, was now in such 

 position that the slightest hint would enable him to comprehend 

 its true nature. He hastened to repeat his trials, as he had the 

 right to do, never having ceased to occupy himself with a ques- 

 tion which lay at the very heart of his doctrine." 



"On the 24th of June, 1783," continues M. Berthelot, "he 

 repeated the combustion of hydrogen in oxygen, and he obtained 

 a notable quantity of water without any other product, and he 



I Watt, "Correspondence," p. 31. 



