204 THE SCIENTIFIC PAPERS OF 



the carbon refused to take up oxygen, or the hydrogen refused to 

 take oxygen, and what had been called by Bunsen, and shortly 

 after him, by St. Claire Deville, dissociation, arose. The point 

 of dissociation was not a fixed one ; partial dissociation came 

 into play at a comparatively low temperature, and went on 

 increasing at a higher temperature in very much the same 

 ratio as vapour density increased with temperature. Thus, 

 if aqueous vapour were passed through a tube at a sufficient 

 temperature the whole of the vapour would be dissociated, and the 

 oxygen and the hydrogen would be separated. It was true if these 

 gases were left to themselves they would, the moment the tem- 

 perature lowered, again associate or burn ; but if precautions were 

 taken to cool them rapidly after they had attained that high 

 temperature they would be found as a mixture of oxygen and 

 hydrogen simply. The author had stated that the law which 

 governed these actions was not well known and required research, 

 but Dr. Siemens would like to know whether he was aware of the 

 researches of St. Claire Deville on the subject. It might be that 

 the determinations of St. Claire Deville were not quite correct, but 

 in the meantime they might be regarded as being so. He found 

 that at atmospheric pressure the point of half dissociation of 

 aqueous vapour arose at a temperature of 2,800 Centigrade, and 

 that of complete dissociation at a much higher temperature. Taking 

 that law as determined by the French philosopher, it did seem 

 reasonable to suppose that when a mixture of hydrogen and 

 oxygen, with or without a mixture of nitrogen, exploded, the point 

 was reached beyond which the temperature did not increase, and, 

 according to the author, that point was 1,500 Centigrade. If such 

 a temperature was reached in the working cylinder complete 

 combustion would not take place immediately, but only partial 

 combustion would occur, which would go on as the temperature 

 diminished by absorption into the cylinder or by expansion, and 

 that combustion would be completed only in the course of the 

 stroke. In that way the action which had been described with 

 reference to the diagrams was reasonable enough. With regard to 

 the mechanical arrangement of gas-engines, the author distin- 

 guished between three types. In the first, the mixture of gas 

 and air drawn in at atmospheric pressure was exploded. In the 

 second, with which the author had connected Dr. Siemens's name 



