290 Messrs. B. Silliman and H. Wurtz on Flame Temperatures, 



pletion of the change in the gas's density, by which a new posi- 

 tion of equilibrium will be obtained — during this time we shall 

 have the gas acted upon by an external force ±tt ; or, to keep 

 to the language of the first part of this article, we shall have an 

 external force doing internal work and increasing total energy 

 if 7r is positive; and if ir is negative, we shall have a part of our 

 internal force, or potential energy, consumed in doing external 

 work, generating vis viva in outside particles, and therefore a loss 

 in total energy. Temperature is raised within the gas in the 

 first place, it is lowered in the second. It is to be observed 

 that it (that is, ±(P— p) and not + P) is the measure, accord- 

 ing to my estimate, of the force which, by a given compression, 

 will develope a given quantity of heat. In neither of these cases 

 is the motion of the piston uniform ; and we shall conclude that 

 when a weight is raised or lowered, by changes in the density or 

 temperature of a gas, by a motion other than uniform, the forces 

 are not of the same name with their effects, and the total energy 

 or the temperature of the gas is altered. But another thing must 

 also be remarked which appears to follow from this, and which, 

 if true, is subversive of a great deal in the modern speculations 

 upon this subject which has hitherto been hardly called in ques- 

 tion. It is this: the alteration of temperature caused by the 

 action of forces of different name from their works must be of 

 very short duration ; and the whole effect, whether of heating 

 or cooling, is neutralized as soon as the piston attains a second 

 position of equilibrium. A good instance of this is given in Mr. 

 Joule's experiments on the cooling of gas by explosion. In 

 those experiments, as soon as the communication was opened 

 between the full and the empty vessels, the full vessel was an 

 example of the internal forces of the gas doing external work. 

 The gas was necessarily cooled in consequence. The empty 

 vessel was a case, as soon as there was any gas in it, of external 

 force doing work within that gas, which was therefore warmed. 

 But as soon as the density had become uniform throughout, 

 which it does in an exceedingly short time, there was found to be 

 no loss or gain of heat. 



Milland, Liphook, March 14, 18/0. 



XL. Investigation of Flame-temperatures, in their Relations 

 to Composition and Luminosity. — First Memoir. By B. Sil- 

 liman and Henry Wurtz *. 



Calorific Powers or Effects of Gases. 



THESE subjects lie, in our belief, at the very basis of the 

 true theory of the phenomena of luminiferous gases, and 

 have practical bearings that can scarcely be overrated. 



* Communicated by the Authors, having been read to the American As- 

 sociation at Salem, August 1869. 



