Sir William Thomson on Vortex Atoms. 15 



The specific gravity of the vapour of water calculated from 

 these elements is 



X. . . . . 0-629 

 XI 0622 



It is necessary to know the exact temperature to which the 

 vapour was heated, since, as is well known, the specific gravity 

 of vapours depends directly on the temperature. This is best 

 calculated from the weight of the dry atmospheric air ¥ l con- 

 tained in the vessel III., whose volume V is known from the ca- 

 libration, while the height of the barometer has been noted at 

 the closing of the vessel. 



If we call the coefficient of expansion of air u, and that of the 

 glass ft, then 



0-76 x 773 P 1 _. 



— A, 



VP 



and the temperature sought is 



/3-aA 



II. On Vortex Atoms. 

 By Professor Sir William Thomson, F.R.S.* 



AFTER noticing Helmholtz's admirable discovery of the law 

 of vortex motion in a perfect liquid — that is, in a fluid per- 

 fectly destitute of viscosity (or fluid friction) — the author said that 

 this discovery inevitably suggests the idea that Helmholtz's rings 

 are the only true atoms. For the only pretext seeming to justify 

 the monstrous assumption of infinitely strong and infinitely rigid 

 pieces of matter, the existence of which is asserted as a probable 

 hypothesis by some of the greatest modern chemists in their 

 rashly-worded introductory statements, is that urged by Lucre- 

 tius and adopted by Newton — that it seems necessary to ac- 

 count for the unalterable distinguishing qualities of different 

 kinds of matter. But Helmholtz has proved an absolutely un- 

 alterable quality in the motion of any portion of a perfect liquid 

 in which the peculiar motion which he calls " Wirbelbewegung " 

 has been once created. Thus any portion of a perfect liquid 

 which has " Wirbelbewegung " has one recommendation of Lu- 

 cretius's atoms — infinitely perennial specific quality. To gene- 

 rate or to destroy " Wirbelbewegung " in a perfect fluid can only 

 be an act of creative power. Lucretius's atom does not explain 



* Communicated by the Author, having been read before the Royal 

 Society of Edinburgh. 





