172 PEINCIPLES OF CHEMISTRY 



most probably it increases), were the combustion concentrated at one 

 point 39 (but it occurs as a flame), were there no loss from radiation and 

 heat conduction, and, chiefly, did dissociation not take place that is, did 

 not a state of equilibrium between the hydrogen, oxygen, and water come 

 about then it would be possible to calculate the temperature of tlie flame 

 of detonating gas. It would then be 10000 . 40 In reality it is very 

 much lower, but it is nevertheless higher than the temperature attained 

 in furnaces and flames, and reaches up to 2000. The explosion of 

 detonating gas is explained by this high temperature, because the 

 aqueous vapour formed must occupy a volume at least 5 times greater 

 than that occupied by the detonating gas at the ordinary temperature. 

 Detonating gas emits a sound, not only as a consequence of the 

 commotion which occurs from the rapid expansion of the heated vapour, 

 but also because it is immediately followed by a cooling effect, the 

 conversion of the vapour into water, and a rapid contraction. 41 



forming the essence of the physical relations, and even without the relations of the entire 

 masses of molecules evincing themselves in purely mechanical relations, inasmuch as 

 an individual atom is something unreal and fantastic. A mechanical change may be 

 imagined without a physical change, and a physical without a chemical change (although 

 such a representation would be artificial), but it is impossible to imagine a chemical 

 change without a physical and mechanical one, for without them we should not perceive 

 it, and through them we attain it. There was a time when the province of physics 

 embraced the whole of chemistry and mechanics. In the present day they have been de- 

 veloped independently and been isolated from each other, but in the future a fresh conjunc- 

 tion is imminent, and is heralded by the laws of the conservation of matter and of energy. 

 59 The flame, or locality where the combustion of gases and vapours is accomplished, 

 is a complex phenomenon, ' an entire factory,' as Faraday says, and therefore we will 

 consider flame in some detail in one of the following notes. 



40 If 34500 units of heat are evolved in the combustion of 1 part of hydrogen, and 

 this heat is transmitted to the resulting 9 parts by weight of aqueous vapour, then we 

 find that, taking the specific heat of the latter as 0'475, each unit of heat raises the 

 temperature of 1 part by weight of aqueous vapour 2'1 and 9 parts by weight (2'l-*-9) 

 0-23 ; hence the 34500 units of heat raise its temperature 7935. If detonating gas is 

 converted into water in a closed space, then the aqueous vapour formed cannot expand, 

 and therefore, in calculating the temperature of combustion, the specific heat at a con- 

 stant volume must be taken into consideration ; it is 0'36 for aqueous vapour. This 

 figure gives a still higher temperature for the flame. In reality it is much lower, but the 

 results given by different observers are very contradictory (from 1700 to 2400), the 

 discrepancies depending on the fact that flames of different sizes are cooled by radiation 

 to a different degree, but mainly on the fact that the methods and apparatus (pyro- 

 meters) for the determination of high temperatures, although they enable relative 

 changes of temperature to be judged, are of little use for determining their absolute 

 magnitude. By taking the temperature of the flame of detonating gas as 2000, I give, 

 I think, the average of the most trustworthy determinations. 



41 It is evident that not only hydrogen, but every other combustible gas, will give an 

 explosive mixture with oxygen. For this reason coal-gas mixed with air explodes 

 when the mixture is ignited. The pressure obtained in the explosions serves as the 

 motive power of gas engines. In this case advantage is taken, not only of the pressure 

 produced by the explosion, but also of that contraction which takes place after the 

 explosion. On this is based the construction of several motors, of which Lenoir's was 



