HOW DISCOVERIES ARE MADE 123 



a discharge was concentrated on a point at the focus of 

 the metallic mirror. 



Moreover, if an object of any kind were placed at the 

 focus, and submitted to the discharge, it became intensely 

 hot; or if it could move if, for instance, it formed the 

 vanes of a little wheel or windmill the wheel revolved 

 rapidly as if it were being bombarded by infinitesimally 

 small bullets. Crookes imagined that by being thus 

 highly rarefied, the gaseous matter changed so as to 

 become ' ultra-gaseous,' that it changed its state in some- 

 what the same manner as ice becomes water or as water 

 becomes steam. 



It is interesting here to recall how Sir William Crookes 

 came to make these most remarkable discoveries. He 

 began by using a spectroscope to investigate the spectrum 

 or coloured light given out by the various constituents 

 into which he had analysed the dust which deposits in 

 the flues used to convey the sulphurous acid produced by 

 the burning of pyrites (a compound of sulphur and iron), 

 then (in the 'sixties) recently introduced as a source of 

 sulphur for the manufacture of sulphuric acid or oil of 

 vitriol. One of his precipitates, when examined with the 

 spectroscope, showed the presence of a bright green light ; 

 and this was traced to the presence of a new element, to 

 which he gave the name 'thallium,' from the Greek 

 thallos, a green twig. 



One of the first things done with a new element is to 

 try to discover its ' equivalent' that is, the proportion by 

 weight of the element which will combine with 8 parts 

 by weight of oxygen. (The number 8 is chosen, because 

 8 parts by weight of oxygen combine with 1 part of 

 hydrogen to form water.) The weighings require to be 

 very accurately made; and a peculiarity which affects all 

 attempts to weigh very accurately must now be told of. 

 The question is often asked as a catch : ' Which weighs 



