THOMSON'S METHOD OF CHEMICAL ANALYSIS 49 



particles it is able to set free a quantity of energy out of all 

 proportion to its mass ; it is this Kinetic Energy or power of 



tYllP' 



doing work, , which may be made appreciable by suf- 

 ficiently increasing the velocity factor v, although the mass 

 factor m may be inconceivably small. It is on this account that 

 the helium molecule of mass 6 x io~ 24 of a gramme, when moving 

 with a velocity 2 x io 9 cm., i.e. about 100,000 miles per second, 

 is capable of causing a flash of light appreciable to the eye when 

 it strikes a fluorescent screen. 



The novel and remarkable method of chemical analysis which 

 is the subject of this article depends upon the fact that if we can 

 communicate high enough velocities to molecules they will be 

 able to produce appreciable and permanent effects when falling 

 upon suitable material ; also upon the fact that if such moving 

 molecules can be electrically charged they become amenable to 

 externally applied electric and magnetic forces and by their 

 movements under these forces can be made, in a phrase, to 

 weigh themselves. The method, indeed, is different from all other 

 chemical methods of determining molecular mass, in that it 

 deals with the individual molecule and not with large numbers. 



It is the outcome of a long and exhaustive series of researches 

 upon the nature of Positive Electricity which Professor Sir 

 J. J. Thomson has been pursuing almost continually since he 

 revolutionised modern views on electricity by his classical 

 experiment with cathode rays, from which he inferred that 

 negative electricity occurs as definite units — corpuscles or 

 electrons — the mass of which is one eighteen-hundredth part of 

 that of an atom of hydrogen. The principal field of these 

 researches has lain in the so-called " Canalstrahlen " or Rays of 

 Positive Electricity which Goldstein, as long ago as 1886, 

 observed in a vacuum tube provided with a perforated cathode. 



These rays were investigated afterwards by Wien, who 

 showed that some of them at least carried a positive charge 

 and had a mass of molecular order : it has, however, been the 

 task of the head of the Cavendish Laboratory to explore, in 

 a detailed and accurate manner, this wide and complex field of 

 research. The subject of the present article is but a single off- 

 shoot of the work. It will be of interest to those who are 

 unable to follow the original papers on the subject to know the 

 method by which it has been demonstrated that just as light 

 4 



