TEMPERATUKE OF IGNITION. 395 



observations are very important as regards the study of fire 

 damp. 



15. The oxidation of gases and of organic substances heated 

 to 300 or 400 may be slowly effected, with a phosphorescent 

 glow, only visible in the dark, such as is seen when ether or 

 absolute alcohol is poured on a red-hot brick. The very products 

 of the oxidation are thus changed, as aldehyde is formed by 

 means of ether. If, however, these reactions are prolonged, 

 especially in the presence of a porous body of small mass, the 

 oxidation is rendered more active by the very heat it liberates, 

 and it may raise the temperature of the system up to the sudden 

 and explosive degree of ignition. This happens sometimes with 

 cotton impregnated with oil, with slowly burning tinder, with 

 brown coal, etc. It has been observed that in manufactories 

 and powder magazines serious accidents have been caused by 

 this cause of ignition, that is to say, due to the elevation 

 of temperature, owing to slow oxidation, which gradually 

 accelerates. 



16. Gases containing sulphur ignite at much lower tempera- 

 tures than hydrocarbon gases, from 250 for example. With 

 reference to this matter may be instanced the following experi- 

 ment. Taking two flat dishes, ether is poured into one and 

 carbon disulphide into the other. If, then, a piece of red-hot coal, 

 but emitting no flame, be introduced into the ether it is extin- 

 guished ; but if it be only rolled in it so as to make the super- 

 ficial incandescence disappear, and we introduce it at once into 

 the carbon disulphide, the latter becomes ignited and can then 

 ignite the ether placed beside it. 



Certain compounds, such as chlorinated and brommated 

 acetylene, ignite spontaneously in contact with air in consequence 

 of analogous phenomena. The same is the case with several 

 phosphoretted compounds. 



17. Let us now return to the question of pressures. Instead 

 of burning a combustible gas by pure oxygen, we should be 

 inclined to expect some advantage arising from nitrogen mon- 

 oxide or nitric oxide, seeing that by their own decomposition 

 these gases supply an additional volume of nitrogen and a 

 supplementary quantity of heat. Nevertheless these advantages 

 are nearly compensated by the necessity of heating the nitrogen 

 (Table on p. 387). 



18. It would be quite another thing if we only considered 

 the total work, for since this is proportional to the heat 

 liberated it is increased with nitrogen monoxide and nitric 

 oxide. 



There also exist certain oxidising solids, such as potassium 

 chlorate, which supply more heat than free oxygen. On the 

 other hand, pure oxygen produces more than poljassium nitrate, 

 and more than most of its liquid or solid compounds. 



