April 7, 1898] 



NATURE 



549 



Priestley proceeded to do. It appears from a letter to Wedg- 

 wood that he repeated Cavendish's experiment during the March 

 of 1783. It will be remembered that he was at this period 

 engaged on his experiments on the seeming conversion of water 

 into air. He had obtained a number of contradictory results 

 which had led Wedgwood, as far back as the previous January, to 

 put certain sagacious queries, which doubtless in the end had their 

 effect in opening Priestley's eyes to the origin of his mistake. 

 But at the time both he and Watt were seeking for fresh evidence 

 to substantiate the possibility of this conversion. Now just as 

 Cavendish thought that Warltire's experiment might throw light 

 upon the particular matter on which he was engaged, so Priestley 

 considered that Cavendish's work might afford evidence, indirect 

 it is true, but still evidence, of the intimate connection between 

 water and air. Cavendish had, he thought, established the 

 converse of the proposition which he and Watt were seeking to 

 prove in showing that "air," or rather certain kinds of " air," 

 could be converted into water weight for weight. It was no 

 longer the original Warltire experiment of exploding common 

 air and hydrogen. Cavendish had indicated the particular kinds 

 which were really concerned in the phenomena, and it was the 

 Cavendish experiment, pure and simple, which he proceeded to 

 repeat. This is obvious from what he says : " Still hearing of 

 many objections to the conversion of water into air, I now gave 

 particular attention to an experiment of Mr. Cavendish's con- 

 cerning the reconversion of air into water by decomposing it in 

 conjunction with inflammable air." Priestley here used the 

 word " decomposing " in a sense contrary to that which the con- 

 text implies ; but that he is consistent in so using it is evident 

 from what follows, and also from similar expressions to be found 

 in his correspondence. But although he professed to repeat 

 Cavendish's experiment, he neglected to do so in Cavendish's 

 manner. He says : " In order to be sure that the water I might 

 find in the air was really a co7istititent part of it, and not what 

 it might have imbibed after its formation \i.e. by contact with 

 the water of the pneumatic trough], I made a quantity of both 

 dephlogisticated and inflammable air, in such a manner as that 

 neither of them should ever come into contact with water, re- 

 ceiving them as they were produced in mercury ; the former from 

 nitre, and in the middle of the process (long after the water of 

 crystallisation was comp over), and the latter from perfectly made 

 charcoal. The two kinds of air thus produced I decomposed by 

 firing them together by the electric explosion, and found a 

 manifest deposition of water, and to appearance in the same 

 quantity as if both the kinds of air had been previously confined 

 by water. 



" In order to judge more accurately of the quantity of water so 

 deposited, and to compare it with the weight of the air decom- 

 posed, I carefully weighed a piece of filtering. paper, and then 

 having wiped with it all the inside of the glass vessel in which 

 the air had been decomposed, weighed it again, and I always 

 found, as nearly as I could judge, the weight of the decomposed 

 air in the moisture acquired by the paper. ... I wished, how- 

 ever, to have had a nicer balance for the purpose : the result was 

 such as to afford a strong presumption that the air was recon- 

 verted into water, and therefore that the origin of it had been 

 ivater." 



These passages, when compared with the accounts given of 

 his own work by Cavendish, strikingly exemplify the difference 

 in the character of the two experimentalists. It would be 

 difficult to pack a greater number of errors into a couple of 

 paragraphs than are contained in these sentences. The ex- 

 pressions in italics show that Priestley wholly failed to compre- 

 hend the true origin of the water. In his laudable anxiety to 

 free the two gases from extraneous moisture, he committed 

 blunder after blunder. His method of obtaining the oxygen was 

 bad ; that of procuring the inflammable air was worse. Both 

 the gases must have been highly impure, and it was a physical 

 impossibility that they should have given their aggregate weight 

 in water, even after making every allowance for Priestley's crude 

 and imperfect method of determining it. 



Bad, however, as the experimental work was, what it appeared 

 to teach was not lost on Watt : it clearly proved to him that 

 water and air were mutually convertible. How the theory took 

 shape in his mind is evident from the terms in which the two 

 series of Priestley's experiments are coupled together in his 

 letters to Gilbert Hamilton, to De Luc and to Black. Each set 

 is regarded as complementary to the other, and, both taken to- 

 gether, are held to prove that air and water are mutually con- 

 vertible, and are therefore essentially the same. Under date 



NO. 1484, VOL. 57] 



April 21, 1783, he tells Black that " Dr. Priestley has made 

 more experiments on the conversion of water into air, and I 

 believe I have found out the cause of it ; which I have put in 

 the form of a letter to him, which will be read at the Royal 

 Society with his paper on the subject." He then proceeds to 

 give Black a summary of the three sets of facts, or supposed facts, 

 on which he bases his generalisation, and he makes use of these 

 significant words : " In the deflagration of the inflammable and 

 dephlogisticated airs, the airs unite with violence — become red- 

 hot —and on cooling, totally disappear. The only fixed matter 

 which remains is water ; and water, light and heat are all the 

 products. Are we not, then, authorised to conclude that water 

 is composed of dephlogisticated and inflammable air, or 

 phlogiston, deprived of part of their latent heat, and that 

 dephlogisticated, or pure air, is composed of air deprived of its 

 phlogiston, and united to heat and light ; and if light be only a 

 modification of heat or a component part of phlogiston, then 

 pure air consists of water deprived of its phlogiston and its latent 

 heat." Very similar turns of expression and trains of reasoning 

 are to be met with in other letters to his friends, written at about 

 the same period. In all it is abundantly clear that, whatever 

 may have been his surmises as to the real nature of water, it was 

 the conception of the mutual convertibility of air and water that 

 was uppermost in his mind. These passages, however, constitute 

 Watt's claim to be regarded as the true and first discoverer of 

 the compound nature of water. 



Three days after the letter to the Royal Society was written, 

 or rather dated, there came a bolt from the blue in the form of a 

 letter from Priestley to Watt. " Behold," it said, " with surprise 

 and with indignation the figure of an apparatus that has utterly- 

 ruined your beautiful hypothesis, and has rendered some weeks ■ 

 of my labour in working, thinking, and writing almost useless."' 

 The doubts of Wedgwood, certainly no mean authority on the 

 properties of baked clay, had, in fact, led Priestley to devise an> 

 experiment by which it was proved beyond all doubt that this - 

 seeming conversion of water into air was really due to an inter- 

 change of steam and air, effected by diffusion through the porous 

 material of the retort. Well might Priestley cry to De Luc, 

 " We are undone ! " Watt's faith in the " beautiful hypothesis " ' 

 was no doubt rudely shaken, but it was not shattered. In his 

 answer to Priestley he denied that it was ruined : "It is not 

 founded," said he, " on so brittle a basis as an earthen retort." 

 Priestley, however, would have none of it : theories with him — 

 always excepting the all -comprehensive one of phlogiston, which 

 was the head and front of his creed, as, indeed, of his subsequent 

 offending — had at no time much value, for, as Marat said of 

 Lavoisier, he abandoned them as readily as he adopted them^ 

 changing his systems as he did his shoes. Indeed, he rather 

 prided himself on his capacity for quick change. " We are, at 

 all ages," he once said, "but too much in haste to understand, SiS 

 we think, the appearances that present themselves to us. If we 

 could content ourselves with the bare knowledge of new facts,,, 

 and suspend our judgment with respect to their causes, till by 

 their analogy we were led to the discovery of more facts, of a 

 similar nature, we should be in a much surer way to the attain- 

 ment of real knowledge." With a candour all his own, he im- 

 mediately added : " I do not pretend to be perfectly innocent in^ 

 this respect myself; but I think I have as little to reproach* 

 myself with on this head as most of my brethren ; and whenever 

 I have drawn general conclusions too soon, I have been very 

 ready to abandon them. ... I have also repeatedly cautioned 

 my readers, and I cannot too much inculcate the caution, that 

 they are to consider new facts only as discoveries, and mere 

 deductions from these facts, as of no kind of authority ; but to 

 draw all conclusions, and form all hypotheses, for themselves." 



Watt's mind was of a very different cast. He did not lightly 

 adopt opinions ; his convictions were slowly and deliberately 

 formed, and were retained with a corresponding tenacity. But^ 

 all the same, he eventually thought it prudent to withdraw his^ 

 letter ; and three days prior to the reading of Priestley's paper^ 

 which accompanied it, Priestley informed Sir Joseph Banks of 

 Watt's desire that the letter should not be publicly read. That 

 it was withdrawn on account of what Watt calls Priestley's 

 "ugly experiment," is stated by him in a letter to Black, on the 

 ground that this experiment rendered " the theory useless in- 

 so far as relates to the change of water into air. ... I have 

 not given up my theory [that is, as to the mutual convertibility 

 of water into air], though neither it nor any other known one 

 will account for this experiment." 



In the meantime Cavendish had been pursuing his inquiries. 



