CH. xxxv. FREE MOLECULES IN VACUUM TUBES. 365 



a series of vortices in and between these molecules, and is 

 the best solution at which mathematicians have yet arrived 

 on the subject. 



Passage of Free Molecules in Vacuum Tubes. 

 Meanwhile Mr. Crookes was able in 1877 to show by actual 

 experiment the movement of the molecules of a gas in so- 

 called 'vacuum tubes.' It is well known that even in the 

 most perfect vacuum which we can obtain in a glass tube, 

 an enormous number of molecules of gas or air always 

 remain, and it is in consequence of the movements of these 

 molecules under the influence of light or heat that a kind of 

 windmill can be made to turn inside those curious glass 

 bulbs, called by Mr. Crookes 'radiometers.' He has now 

 shown that in very high vacua about the millionth of an 

 atmosphere in consequence of there being comparatively 

 few molecules in the bulb, their mean free path is long 

 enough to enable them to travel across without materially 

 interfering with each other. In this free state most curious 

 effects are produced by currents of electricity. The mole- 

 cules driven from the negative pole hit with such force 

 against the glass that they make it glow with phosphor- 

 escence, and substances such as the diamond and the ruby, 

 which are scarcely phosphorescent under ordinary circum- 

 stances, if placed in the path of these molecules, shine with 

 extraordinary brilliancy. But the point of most importance 

 is that the gas at this high exhaustion behaves quite differ- 

 ently from ordinary gases, owing probably to the greater 

 freedom of molecular action, and Mr. Crookes believes that 

 it will be found to torm a fourth state of matter, as distinct 

 from the gaseous as the gaseous is from the liquid state. 



It may perhaps seem strange that, while speaking of 

 these discoveries of theoretical interest in physics, we omit 

 the description of the numerous inventions which have been 



