N AND ITS SALINE COMBINATIONS 169 



in aiiy j;< but also make a number of experiments explaining the 

 phenomenon of combustion. 



Thus, for example, it may be demonstrated, by the aid of the 

 eudiometer, that for the ignition of detonating gas a definite temperature 

 is required. If the temperature be below that required, combination 

 will not take place, but if at any spot within the tube it rises to the 

 temperature of inflammation, then combination will ensue at that spot, 

 and evolve enough heat for the ignition of the adjacent portions of the 

 detonating mixture. If to 1 volume of detonating gas there be added 

 10 volumes of oxygen, or 4 volumes of hydrogen, or 3 volumes of 

 carbonic anhydride, then we shall not obtain an explosion by passing 

 a spark through the diluted mixture. This depends on the fact that 

 the temperature falls with the dilution of the detonating gas by another 

 gas, because the heat evolved by the combination of the small quantity 

 of hydrogen and oxygen brought to incandescence by the spark is not 

 only transmitted to the water proceeding from the combination, but 

 also to the foreign substance mixed with the detonating gas. 34 The 

 necessity of a definite temperature for the ignition of detonating gas is 

 also seen from the fact that pure detonating gas explodes in the presence 

 of a red-hot iron wire, or of charcoal so feebly incandescent as to be 

 hardly distinguishable by day light, but with a lower degree of in- 

 candescence there is not any explosion. It may also be brought about 

 by rapid compression, when, as is known, heat is evolved. 3 '" 1 Experi- 

 ments made in the eudiometer showed that the ignition of detonating 

 gas takes place at a temperature between 450 and 500 . 36 



exactly the above proportion neither one nor the other remained. The composition of 

 water was thus definitely confirmed. 



55 Concerning this application of the eudiometer, see the chapter on nitrogen. 



31 Thus \ volume of carbonic oxide, an equal volume of marsh gas, two volumes of 

 hydrogen chloride or of ammonia, and six volumes of nitrogen or twelve volumes of air 

 added to one volume of detonating gas, prevent its explosion. 



"" If the compression be brought about slowly, so that the heat evolved succeeds in 

 passing to the surrounding space, then the combination of the oxygen and hydrogen does 

 not take place, even when the mixture is compressed by 150 times ; for the gases are not 

 heated. If paper soaked with a solution of platinum (in aqua regia) and sal ammoniac 

 be burnt, then the ash obtained contains very finely-divided platinum, and in this form 

 it is best fitted for setting light to hydrogen and detonating gas. Platinum wire requires 

 to be heated, but platinum in so finely divided^ a state as it occurs in this ash inflames 

 hydrogen, even at 20 3 . Many other metals, such as palladium, iridium, and gold, act 

 with a slight rise of temperature, like platinum ; charcoal, like the majority of finely 

 divided substances, inflames detonating gas at 850, but mercury, at its boiling point, 

 I..,. s not inflame detonating gas. All data of this kind show that the explosion of 

 detonating gas presents one of the many cases of contact phenomena. 



58 From the very beginning of the diffusion of the idea of dissociation, it might have 

 been imagined that reversible reactions of combination (the formation of Ho and O 

 belongs to this number) start at the same temperature as that at which dissociation 

 begins. And so it is in many cases, but not always, as may be seen from the facts (1) that 



