AND THE CONSTITUTION OF ELASTIC FLUIDS. HI 



their axes at the moment when the body becomes fluid or 

 aeriform, or from the loss of rapidity of vibration in con- 

 sequence of the motion of the particles through greater 

 space."* I have myself endeavoured to prove that a rotary 

 motion, such as that described by Sir H. Davy, will account 

 for the law of Boyle and Mariotte. and other phenomena 

 presented by elastic fluids ; f nevertheless, since the hypo- 

 thesis of Herapath, in which it is assumed that the particles 

 of a gas are constantly flying about in every direction with 

 great velocity, the pressure of the gas being owing to the 

 impact of the particles against any surface presented to 

 them, is somewhat simpler, I shall employ it in the follow* 

 ing remarks on the constitution of elastic fluids ; premising, 

 however, that the hypothesis of a rotary motion accords 

 equally well with the phenomena. 



Let us suppose an envelope of the size and shape of a 

 cubic foot to be filled with hydrogen gas, which, at 60° 

 temperature and 30 inches barometrical pressure, will 

 weigh 36'927 grs. Further, let us suppose the above quan- 

 tity to be divided into three equal and indefinitely small 

 elastic particles, each weighing 12*309 grs. ; and further, 

 that each of these particles vibrates between opposite sides 

 of the cube, and maintains an uniform velocity except at 

 the instant of impact; it is required to find the velocity at 

 which each particle must move so as to produce the atmo- 

 spherical pressure of 14,831,712 grs, on each of the square 

 sides of the cube. In the first place, it is known that if 

 a body moving with the velocity of S2^ feet per second be 

 opposed, during one second, by a pressure equal to its 

 weight, its motion will be stopped, and that, if the pressure 

 be continued one second longer, the particle will acquire 



* Elements of Chemical Philosophy, p. 95. 



t Mr. Rankine has given a complete mathematical investigation of the 

 action of vortices, in his paper on the Mechanical Action of Gases and 

 Vapours Trans. R. S, Edin., vol. xx. part 1 — May, 1851, J. P. J. 



