222 Oil the Nature of what is conDiionly termed a '' Vacuum.^'' 



i. e. the correction is about '88 R, where R is the half-side. It 

 is quite possible to calculate the correction, according to the 

 preceding principles, on the hypothesis of spherical divergence ; 

 but the result always comes out mach less than the experi- 

 mental value, even if a considerable allowance is made for the 

 shortness of the pipe compared with the wave-length. 



Errata in Note 6 (Pliil. Mag. August). 



Page 12o, line 6 from foot of page, for vortical read vertical. 



— — — 5 from foot of page, for vertical read vortical . 



dS dS 



— 131, line 17, at the end of the line, for -g^ read 02 • 



XXVIII. On the Nature of what is commonly teinned a 

 " Vacuum..'''' 



To the Editors of the Philosophical Magazine and Journal. 



Queen's University, Dublin, 



Gentlemen, August 19, 1877. 



THE readers of Mr. Preston's paper in this month's Num- 

 ber of the ' Philosophical Magazine,' ^^ On the Nature of 

 what is commonly termed a ' Vacuum,' " might perhaps sup- 

 pose with him that the subject is one which had not been pre- 

 viously noticed, and conclude that we are as yet without an 

 explanation of " Crookes's force," in which the vast multitude 

 of the gaseous molecules that are present has been taken into 

 account. 



The subject is one which cannot, I should think, have been 

 overlooked by any real student of the molecular theory of 

 gases ; and in particular your readers will find it thus treated 

 in a paper that I presented ten vears ago to the Roval Society 

 (see Phil. Mag. [IV.] vol. xxxvi. p. 141):— ''It is therefore 

 probable that there are not fewer than something like a unit- 

 eighteen of molecules" [i. e. 1,000000,000(300,000000] "in 

 each cubic millimetre of a gas at ordinary temperatures and 

 pressures. Hence we may see how entirely remote from a state 

 of emptiness that which usually passes under the name of a 

 vacuum-chamber really is. If there be a unit-eighteen of 

 molecules in every cubic millimetre of the air about us, there 

 Avill remain a unit-fifteen " [i. e. a thousand millions of mil- 

 lions] " in every cubic millimetre of the best vacuums of our 

 ordinary air-pumps. The molecules are still closely packed, 

 within about an eighth-metre of one another ; i. e. there are 

 about sixty of them in a waA'e-length of orange light." And 

 in two papers published in last year's ' Philosophical Magazine ' 



