126 Mr Petrie on Spontaneous 



ing to the most probable theory) constitutes the temperature 

 of the gas — becomes increased. This increase of temperature 

 being ultimately the exact thermic equivalent of the original 

 dynamic force of compression mentioned above, minus only 

 the force spent in sound, and such grosser vibrations at the 

 ^arly part of the jet. 



What time is necessary for a given part (Jialf, for instance,) 

 of the motion of a jet of a given magnitude, to descend, as it 

 were, to the ultimate atoms, is a question on which we cannot 

 form even a vague opinion previous to a direct experiment 

 for the purpose. It may be but a few seconds, possibly a few 

 hours, but more probably a few minutes, from any jet of 

 ordinary sizes. It seems clear that the more minute sub- 

 division of the currents requires the longer time; and it 

 appears to be a question of pure mathematics, which, possibly, 

 the present state of the science is competent to solve from 

 practical data, — What degree of minuteness will the breadth 

 of the subdivided currents or whirls, from a given jet, attain in 

 a given time ; to such an extent that one-half of the dynamic force 

 of the current will be invested in currents not greater than that 

 degree of minuteness ? The formula is not easily produced, 

 but will evidently have the time placed as the " index'' of the 

 quantity. And if the time in which half the dynamic power, 

 from a given jet, becomes developed into heat, be accurately 

 determined by experiment, the said formula will enable us 

 to calculate approximately, the absolute atomic volume of the 

 gas ; and thence (by relative atomic weight and specific 

 gravity) of all substances, a subject of which we are at pre-' 

 sent profoundly ignorant. 



To determine the said time (for the development of heat) 

 experimentally, a thermometer and an ordinary blast will 

 not suffice, as the large volume of air with which the jet soon 

 mingles, reduces the whole difference of temperature to an 

 exceedingly small amount, quite inappreciable by the time 

 the heat is materially developed. And if the air issuing 

 from the jet were confined, as in a flaccid bladder, it would 

 part with its heat to the sides of the retaining material, and 

 be radiated from it as fast as it was developed. But pro- 

 bably some such arrangement as the following might exhibit 



