138 HEATING. 



Thus, we find, the oxygen the heat-giving constituent of 

 the air bears a proportion in volume to that of the nitrogen, as 

 1 to 5 ; there being, in fact, but 20 per cent, of oxygen in atmos- 

 pheric air, and no less than 80 per cent, of nitrogen ; a circum- 

 stance which should never be lost sight of in all that has to do 

 with its admission and application. 



Having shown the composition of coal gas, and also of air, 

 with the weights and volumes of their respective constituents, 

 we now proceed to the ascertaining the separate quantity of oxy- 

 gen required by each of those constituents, so as to effect its per- 

 fect combustion, and produce the largest quantity of available 

 heat ; in other words, to find the " chemical equivalent" or vol- 

 ume of air, required for the saturation of this mixed gas. 



Now, this is to be decided, not by the quantity of air we may 

 admit or force into the furnace, but solely by the faculty with 

 which each of these constituents is endowed of uniting chemically 

 with the oxygen. 



With respect to this power, or faculty of reciprocal saturation, 

 the first great natural law is, that bodies combine in certain fixed 

 proportions only, a remarkable feature in this law, as far as 

 gaseous bodies are concerned, being, that it has reference both to 

 volume and weight ; thus, by their concurrence, establishing the 

 principle which now no longer admits of any doubt. * 



The important bearings of this great elementary principle of 

 proportionate combination cannot be more strikingly illustrated, 

 or its influence rendered more familiar, than in the several com- 



* " L'experience a demontre que, de meme que les elemens se com- 

 .binent dans des proportions fixes et multiples, relativeraent a leur poids, 

 fls se combinent aussi, d'une maniere analogue, relativement a leur 

 volume, lorsqu'ils sont a 1'etat de gaz : en sorte qu'un volume d'un 

 element se combine, ou, avec un volume egal au sien, ou avec 2, 3, 4 et 

 plus de fois son volume d'un autre element a 1'etat de gaz. En com- 

 parant ensemble les phenomenes connus des combinaisons de substances 

 gazeuses, nous decouvrons les memes lois des proportions fixes, que celles 

 que vous venons de deduire de leurs proportions en poids : ce qui donne 

 lieu a une maniere de se representer les corps, qui doivent se combiner, 

 sous des volumes relatifs a 1'etat de gaz. Les degres de combinaisons 

 sont absolument les memes, et ce qui dans 1'une est nomme atome, est 

 dans Vautre apelle volume} 1 Berzelius, vol. iv., p. 549. 



