1824.] Dr. Thomson on Subphosphuretted Hydrogen Gas. 247 



and the quantity of air employed in the blast in a given time. 

 The same rule will hold even in what are called chimney fur- 

 naces; and it is ascertained by experience, that those furnaces 

 of steam-engines through which a greater quantity of air passes 

 in a given time, consume a proportionally less quantity of fuel 

 to produce the same effect. Probably blast-furnaces might be 

 advantageously employed in lessening the quantity of fuel used 

 for those valuable machines. 



Although, according to the foregoing experiment, it appears 

 contrary to my original expectation, that, by volume, oxygen gas 

 has the same specific heat as carbonic acid, it by no means 

 follows that caloric should not be evolved during the formation 

 of the latter by combustion. This formation does not consist of 

 a conversion of oxygen into carbonic acid, but of a union of two 

 ingredients into a compound, having an absolute capacity for 

 caforic equal to one of the ingredients only, namely, the oxygen 

 gas ; consequently the whole absolute heat of the carbon is ren- 

 dered free. 



Article III. 

 On Subphosphuretted Hydrogen Gas. By Dr. Thomson, FRS. 

 (To the Editors of the Annals of Philosophy.) 

 GENTLEMEN, Glasgow, Sept. 9, 1824. 



In the account of subphosphuretted hydrogen gas, inserted in 

 the last number of the Annals, I observe a mistake into which 1 

 had fallen while hastily transcribing the account from my 

 common-place book. I have misstated the specific gravity of 

 hydrogen gas. The statement in page 205 of the last number 

 should have been as follows : 



" Subphosphuretted hydrogen gas is composed of 



1 volume hydrogen gas 0*0694 



0-75 volume phosphorus vapour 0*6250 



0*6944 



So that its specific gravity is reduced from 0*9027 to 0*6944, and 

 it contains just nine times as much phosphorus as hydrogen. 

 It may be reckoned a compound of 4 atoms hydrogen and 

 3 atoms phosphorus." 



Your noticing this error in your next number will much oblige, 



Gentlemen, yours truly, 



Thomas Thomson. 



