27 



paper, with the same care which he has devoted to the correspondence 

 of Watt, he would probably have come to the same conclusion ; 

 at all events he would not have failed to observe one circumstance in 

 it, which would have rendered my statement that Priestley never found 

 the weight of the water equal to the sum of the weight of the gases, less 

 *' inconceivable " than his colleague represents it : he would even have 

 found it possible to demonstrate with some degree of certainty the 

 minimum of the deficiency. 



Priestley, though he says nothing of the weights or volumes of the 

 gases which he burnt, any more than he does of the weight or nature 

 of the fluid which he collected, mentions one circumstance very mate- 

 rial to a due estimate of the value of his experiment : he tells us the 

 means employed by him to obtain his gases pure and dry. He ob- 

 tained, then, his oxygen from nitre, and his inflammable gas from the 

 distillation of well-burnt charcoal. No one knows better than M. Du- 

 mas the products of such a distillation. There is reason to think that 

 if charcoal be well burnt, and so dry that the vapour to be decomposed 

 is small in quantity, and the heat at which it is decomposed strong, 

 the whole product of such a distillation would be carbonic oxide and 

 hydrogen, in equal volumes, each volume of the hydrogen evolved 

 from the water in contact with the ignited charcoal corresponding to 

 half a volume of oxygen, and that half-volume producing one volume 

 of carbonic oxide. In this case, if we call the weight of the hydrogen 

 1*, the weight of water formed in burning it is 9*: the weight of 

 hydrogen and oxygen gas burnt is also 9* ; and the weight of car- 

 bonic oxide forming the other half of the distilled gas, is 14*1 ; and 

 this requires for perfect combustion 8 of oxygen — together = 22'1, 

 which added to 9 = 31*1 ; 5o thai the weight of icater formed in burn- 

 ing such a mixture is to the weight of the gases burnt as 9 : 31 '1. 



But this theoretical result has never been experimentally demon- 

 strated*. In the experiments that have been actually made, in which no 

 particular attention has been paid to the heat at Avhich the decompo- 

 sition is effected, part of the carbonic oxide is replaced by carbonic 

 acid, and part of the hydrogen combines with carbon. Henry analysed 

 this mixed gas in 1808, and found that 100 volumes, freed from the 

 carbonic acid, required 60 of oxygen for perfect combustion, and gave 

 35 of carbonic acid. I shall presently have occasion to mention, that 

 Cavendish's unpublished experiments, made in 1783, furnish nearly the 



* Cruikshank's experiments, giving, as computed by Henry (Nicholson's 

 Journal, vol. xi. p. 71), 43 measures of carb. oxide in 100 of the gas, ap- 

 proach nearest to the theoretical composition. 



