THE KINETIC THEORY OF MATTER. 



149 



shot, the volume would be 3 j2/ir times greater. Probably we have an 

 approach to such a condensation in the liquid and solid states, where the 

 molecules may be regarded as each within the spheres of action of its 

 next neighbours all the time, and we shall therefore assume, as an 

 approximation of the right order, that the volume of a gas containing N 



molecules is 7rs when liquefied. 



If, then, 1 c.c. of gas at 0' 

 volume v of liquid of density 8, 



and 760, and density A, condenses to 



But our equation for the free path is 



0) 



1 (2) 



From (1) and (2) we have s = 6vL. 



We know v and L in a number of cases, and can therefore find s. Then, 

 substituting this value of s in (2), we can find N. The following table 

 shows the results obtained for hydrogen, oxygen, and water vapour. It 

 is hardly necessary to point out that, seeing the assumptions made, the 

 results are only to be taken as indicating the order of magnitude involved. 

 The value of N, for instance, differs for different gases, whereas by 

 Avogadro's Law it should be the same. 



Using the more correct formulae, Maxwell calculated that 1ST for gases 

 at and 760 should be 19 x 10 18 , say 2 x 10 19 (Papers, ii. p. 372), 

 whence m for hydrogen comes out 4'5 -4- 10 24 . The value of N can also 

 be found by an entirely different method from the consideration of 

 the electrical properties of gases. The value so found is 3'9xl0 19 

 (Thomson, Condndion of Electricity through Gases, p. 130). Maxwell 

 further obtained for s with hydrogen the value 5 -f 10 8 . If we assume 

 that this is about the value of the molecular diameter in other cases, an 

 assumption perhaps warranted from the nearness of the values of v for 

 different substances, then this would imply that in solids and liquids the 

 centres of the molecules are a distance apart comparable with 5 -=- 10 8 . 



Forces Acting on unequally heated Surfaces in High Vacua. 



The motions of unequally heated surfaces in rarefied gases were first 

 investigated by Crookes, who was led to the invention of the radio- 

 meter a beautiful illustration of such motions. In a simple form, it 

 consists of four mica vanes, each lampblacked on one face and attached 

 to the four arms of a cross, pivoted to move with very slight friction 

 about the centre (Fig. 79). The lampblacked faces are so placed as all 

 to move round forward or all backward. 



