* 



27 



the lengths of luminiferous waves. To this he was led, 

 not by any attempt at individual measurement, but by 

 taking account of them in the aggregate, and observing 

 the tints which they scatter laterally when clustered in 

 the form of actinic clouds. The small bodies with which 

 experimental Science has recently come into contact are 

 not confined to gaseous molecules, but comprise also com- 

 plete organisms ; and the same philosopher has made a 

 profound study of the momentous influence exerted by 

 these minute organisms in the economy of life. And if, 

 in view of their specific effects, whether deleterious or 

 other, on human life, any qualitative classification, 

 or quantitative estimate be ever possible, it seems 

 that it must be effected by some such method as that 

 indicated above. 



Again, to enumerate a few more instances of the 

 measurement of minute quantities, there are the average 

 distances of molecules from one another in variaus gases 

 and at various pressures ; the length of their free path, or 

 range open for their motion without coming into collision ; 

 there are movements causing the pressures and dif- 

 ferences of pressure under which Mr. Crookes' radio- 

 meters execute their wonderful revolutions. There are 

 the excursions of the air while transmitting notes of high 

 pitch, which through the researches of Lord Rayleigh 

 appear to be of a diminutiveness altogether unexpected. 

 There are the molecular actions brought into play in 

 the remarkable experiments by Dr. Ker, who has suc- 

 ceeded, where even Faraday failed, in effecting a visible 

 rotation of the plane of polarisation of light in its pas- 

 sage through electrified dielectrics, and on its reflexion at 

 the surface of a magnet. To take one more instance, 

 which must be present to the minds of us all, there are 

 the infinitesimal ripples of the vibrating plate in 



