Marsh.] 



120 



[March 6, 



the particles are much nearer together. The first thing, therefore, is to 

 determine how much, if any, of the heat produced may be due to this 

 change of the molecular condition of the air, and how much to the work 

 expended in compressing the air. 



The following very ingenious experiment performed by Joule is con- 

 clusive in showing that the mere change of distance of the molecules 

 of a permanent gas neither produces nor absorbs heat to an apprecia- 

 ble extent. In the figure, we have two strong vessels, of which A 

 contains compressed air, say under the pressure of 20 atmospheres ; B, 

 on the other hand, is a vacuum. The two vessels are connected with each 

 other by a tube having a stop-cock, which we may suppose to be shuk 

 The whole apparatus is plunged into a vessel of water. After the tem- 

 perature of the water has been very accurately ascertained, open the 

 stop-cock, and thus allow both vessels to have the same pressure. 



When the experiment is finished it will be found that there is no change 

 in the temperature of the water. The prevalent idea is that when air 

 expands it becomes colder, and that when condensed it becomes hotter ; 

 but Joule, by this experiment, has shown that no appreciable change of 

 temperature occurs when air is allowed to expand in such a manner as 

 not to develop mechanical power." 



Prof. Tyndall (" Heat considered as a Mode of Motion"— 1863, p. 88,) in 

 introducing this experiment, says : " Is it not possible to allow a gas to 

 expand, without performing work? This question is answered by the 

 following important experiment, which was first made by Gay-Lussac," 

 and, after describing it, he says "We are taught by this experiment that 

 mere rarefaction is not of itself sufficient to produce a lowering of the 

 mean temperature of a mass of air. It was, and is still, a current notion, 

 that the mere expansion of a gas produced refrigeration, no matter how 



