258 PROGRESS OF SCIENCE. 



rapid and powerful motion of the molecules which still 

 remain in the vacuum glass bulb the increased velocity of 

 their motion being caused by the undulatory progress of 

 light. Be this as it may, the radiometer is capable of 

 giving much useful assistance to the photographer for 

 regulating the light (i) in the operating room, so as to 

 obtain negatives of the same quality ; (2) in the dark 

 room, so as to prevent all injury to the sensitive surfaces 

 there exposed. " The instrument is also capable of giving 

 much information in climatology, since the. amount of light 

 received at any given place has its effect upon life and 

 health, vegetable, animal, and human, and its relative 

 amount at any given place is hence a point of no small 

 physiological moment. For we can now ascertain and 

 distinguish how much of the annual temperature of a place 

 is due to the direct influence of the sun alone, and how 

 much to the other factors, such as geological influence, 

 altitude, atmospheric and oceanic currents, warm or cold, which 

 sweep over or near it." In connection with the radiometer 

 Crookes made wonderful experiments ON ATOMS AND 

 MOLECULES BY THE APPLICATION OF ELECTRICITY, which 

 demonstrated the bombardment of particles on one another 

 so that Crookes at one stroke seems to have disclosed 

 and substantiated the soundness of numerous theories which 

 hitherto were held as mere speculations. It should be stated 

 that in 1871, Varley, unknown to Crookes, had made an 

 almost similar discovery, by controlling a molecular current 

 by a magnet. Concurrently also, although not directly 

 connected with Crookes's experiments, the famous physicist 

 Hertz well known for his discovery of a finite velocity 

 of propagation of electro- magnetic action, and his de- 

 monstration of electro-magnetic waves in air, and their 

 reflection worked in the same direction as Varley and 

 Crookes. The one result which surpasses all others in 

 importance, and adds immensely to our knowledge of the 

 elementary forces which govern the universe, is the ascer- 

 taining that atomic energy as shown by Crookes, Thomson, 

 and Tesla is millions of times greater and more powerful 

 than any with which we were acquainted ; for if a stream 



