144 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



one half its volume of oxygen in forming carbonic anhydride, 

 but does not suffer any change of volume in the process, 

 thus: 



carbonic oxide 4- oxygen --> carbonic anhydride 

 i vol. \ vol. i vol. 



Carbonic anhydride, on the other hand, parts with half its 

 oxygen and doubles in volume when it combines with 

 carbon to form carbonic oxide, thus : 



carbonic anhydride + carbon > carbonic oxide. 



I VOl. 2 VOh. 



Dalton decomposes carbonic anhydride by sparking. 



An independent proof of these relationships was obtained 

 by John Dalton of Manchester (1766-1844), who found 

 that carbonic anhydride (18 vols.) was decomposed by the 

 electric spark into carbonic oxide (16 vols.) and oxygen (9 

 vols.). The combustion of carbonic oxide is therefore a 

 reversible process, as shown by the equation : 



Carbonic oxide + oxygen ^ carbonic anhydride 

 i vol. \ vol. i vol. 



Dalton writes : 



"Carbonic acid is decomposed by electricity into car- 

 bonic oxide and oxygen. I assisted Dr. Henry in an 

 experiment in which 52 measures of carbonic acid were 

 made 59 measures by 750 shocks; the gas after being 

 washed became 25 measures, whence these had arisen from 

 the decomposition of 18 measures of acid 1 ; these 25 

 .measures consisted of 16 carbonic oxide and 9 oxygen ; for, a 

 portion being subjected to nitrous gas, manifested 1/3 of its 

 bulk to be oxygen ; and the rest was fired by an electric 



1 59 ~ 2 5 ~ 34 measures of carbonic acid gas remained after sparking, 

 out of the 52 measures originally taken. 



