132 ESSAYS BIOGRAPHICAL AND CHEMICAL 



our knowledge ; the longer waves are the vibrations the 

 discovery of which was due to Hertz, which are produced 

 by electric oscillations, and which are now being utilised 

 for telegraphy without wires. 



It is, however, with the shorter, and not with the longer 

 vibrations, that we have to do. These are not incom- 

 mensurable with the dimensions of a molecule, for the 

 larger molecules are believed to be about the millionth 

 of a millimetre in diameter, or about one hundredth of 

 the shortest wave-length which has been measured. And 

 just as an interposed grating offers little opposition to the 

 course of a large wave of water, while it will stop ripples, 

 so matter is sufficiently fine-grained not to oppose the 

 spread of a Hertzian wave of great wave-length, although 

 it may stop light and other vibrations of shorter wave- 

 length. It is known, indeed, that the signals of wireless 

 telegraphy are not blocked by material obstacles such as 

 houses, or even hills, while a very thin slice of brick or 

 stone is opaque to light. 



When two thin strips of gold-leaf are suspended from a 

 glass support, and given an electric charge, they diverge, 

 owing to the repulsive force between the charges of elec- 

 tricity which they contain. They will remain apart for 

 an indefinite time, provided the charge cannot escape 

 through the support. But on exposure to ultra-violet 

 rays, the electroscope, if charged with negative electricity, 

 is at once discharged, and the leaves fall together. The 

 electricity finds some means of leaving the leaves of gold 

 and they drop, under the action of gravity. The rays do 

 not, however, discharge a positively charged electroscope. 

 This is one of the most characteristic properties of the 

 ultra-violet rays, and, as will shortly be seen, of rays from 

 sources other than luminous bodies. This fact was dis- 

 covered by Hertz. 



The Becquerel family has contributed much to our 



