Dr. Thomson— anrf his Jnswer . 347 



" The ratio, therefore," says Dr. Apjohn, " of the dry gas to 



the expansion produced in it, by the moisture, is tliat of - . ' ■ : 1. 



Pr. Thomson, through some oversight, considers this as the 

 ratio in volume of the dry gas to the steam, with which it is 



saturated, whereas the true ratio of these is that of ^ — ^: 1 ; in- 



P 

 asmuch as a given volume of gas at any temperature saturated 



with moisture, contains as much steam as could exist at that 

 temperature in a vacuum of equal capacity*." In numbers, 

 the first ratio is that of 0.983 to 1 ; the second is 5fi.7 to I ; 

 and 983 : 56.7 :: 1 : 67.7; so that Thomson commits 

 the trijling blunder, on a subject in which he conceives himself 

 supreme master and proficient, of making the volume of vapour 

 in a moist gas about g'^ of the true quantity. 



" We see from these examples," says Thomson in this won- 

 derful paper " on the influence of humidify in modifying the 

 speci/ic gravity of the gases," " that the specific gravity of air is 

 diminished very nearly, by the volume of vapour mixed with it. 

 And the lower the temperature, the more nearly does this ap- 

 proach to accuracy; bcause the specific gravity becomes 

 always less and less considerable." What ignorance of 

 physics ! " The densities of all the rest (except hydrogen,)" 

 says Dr. Apjohn, are diminished by moisture, and the more as 

 we descend on the scale of temperature ; for as we descend, 

 their specific gravities increase, and of course the difference 

 between steam and the constant quantity 0.472. As an infer- 

 ence also from this result, I may remark, that the rule so much 

 insisted upon of taking the specific gravities of the gases at a 

 low temperature, is so far from being general, as to apply to hy- 

 drogen alone t." Now listen to the regius expounder of science. 

 " I have little doubt that the specific gravity of hydrogen gas 

 found by Berzelius and Dulong; namely, 0.0688, was a little 

 too light, in consequence of the presence of the vapour of water 

 in it J." This is much too absurd. Ihe vapour of water is seven 

 times denser than that of hydrogen, and must therefore always 

 increase the specific gravity of that gas. What a pity that 

 Dr. Apjohn published his exposure of the Doctor's notions so 

 early. We should have been favoured ere long from his labo- 

 ratory, with experimental determinations of the specific gra- 

 vities of the gases, saturated with moisture, in which the re- 

 sults would all have been clipped and trimmed to his fantastic 

 tabular numbers, like the grotesque shrubs in a Dutch garden. 



* AnnaU of Phil May 1 822. p. 38*;. t Annals of Phil. p. 387. 



: JnnaU of Phil. April, 1822. 



