66 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



noise of metallic hail.' Thus then it was shown 

 experimentally that there are no such things as abso- 

 lutely permanent gases. 



The kinetic theory of gases, now generally accepted, 

 refers the elasticity of gases to a motion of translation 

 of their molecules, and we are assured that in the case 

 of hydrogen at a temperature of 60 Fahr. they move 

 at an average rate of 6,225 feet in a second ; while as 

 regards their size, Loschmidt, who has since been con- 

 firmed by Stoney and Sir W. Thomson, calculates that 

 each is at most 5-^-0^0 o~o o" f an mcn m diameter. 



We cannot, it would seem at present, hope for any 

 increase of our knowledge of atoms by any improve- 

 ment in the microscope. With our present instruments 

 we can perceive lines ruled on glass of o^^th of an 

 inch apart. But, owing' to the properties of light itself, 

 the fringes due to interference begin to produce confu- 

 sion at distances of 74 ^ 00 , and in the brightest part of 

 the spectrum at little more than 9 1 O th they would 

 make the obscurity more or less complete. If indeed 

 we could use the blue rays by themselves, their waves 

 being much shorter, the limit of possible visibility 

 might be extended to yWo o~o J an( ^ as Helmholtz has 

 suggested, this perhaps accounts for Stinde having 

 actually been able to abtain a photographic image of 

 lines only TS-oVo-o th of an inch apart. It would seem 

 then that, owing to the physical characters of light, we 

 can, as Sorby has pointed out, scarcely hope for any 

 great improvement so far as the mere visibility of struc- 

 ture is concerned, though in other respects no doubt 

 much may be hoped for. At the same time, Dallinger 

 and Royston Pigott have shown that, so far as the mere 



