Temperature and Volume of Gases, 125 



depends on the product of the two. By calculation we find 

 that this maximum effect is absolutely obtained at an expan- 

 sion to about 2 J volumes (when T = 60°), but is very nearly 

 approached when the expansion is only to Ij^^th volumes, — 

 consequently the expansion should in no case be carried fur- 

 ther than this amount in practice. 



On Spontaneous Re heating of Air Cooled by Issuing, in a Jet, 

 from a more Compressed state. (An important fact, if 

 correct, in regard to the Practical Application of such Vari- 

 ations of Temperature.) Also, on the Possible Determination 

 thereby of the Mutual Distances of the Ultimate Atoms of 

 Matter. By William Petrie, Esq., C.E. Communicated 

 by the Author. 



When air is allowed to expand, from a compressed state, 

 by issuing in a jet, the whole mechanical power of its expan- 

 sion (or, what is equal thereto, the mechanical power which 

 may be supposed to have compressed it from a temperature 

 and pressure equal to what it has on re-issuing,) is spent in 

 producing velocity in the expanded air. This velocity takes 

 the form of currents in the surrounding air, which results 

 (as in all fluids) in a slower general motion of a larger l3ody 

 of the surrounding air, along with internal currents in a 

 rotary direction, produced by the mingling of the primary jet 

 with the surrounding air. The slower general motion afore- 

 ' said, and each of the internal rotary currents, again subdivide 

 themselves on the same principle, so that, as time elapses, 

 the primary jet becomes subdivided into an increasing number 

 of currents of a diminishing volume individually, and in more 

 and more promiscuous rotary directions, while the general 

 motion of the jet and the air incorporated with it becomes 

 less as more is incorporated, but represents continually less 

 dynamic force (i.e. m x v^ becomes less) : the dynamic force 

 becoming more and more invested in the secondary currents. 

 This process goes on until the continual subdivision of cur- 

 rents has reduced them to atomic currents ; that is to say, 

 the amounts of motion in the ultimate atoms — which (accord- 



