368 GREAT MEN OF SCIENCE 



or molecules in such a way that they support one another in 

 their external action, and do not become imperceptible ex- 

 ternally by reason of want of order in their arrangement. In 

 the same way, all kinds of ether waves, however different 

 they may be, according to their length, in their peculiarities, 

 effects, and mode of origin, nevertheless only come from 

 matter. Matter is therefore without doubt the source of all 

 electro-magnetic fields, and nevertheless these fields have 

 other qualities at their place of origin in the interior of the 

 atoms, than those revealed after they have escaped outside 

 them. They show inside the atoms pecuUar properties, 

 which go beyond Maxwell's equations. 



As regards the limits of validity, which are thus not 

 wanting also in the case of Maxwell's equations, the course of 

 our progress in knowledge is always such that laws are found 

 and confirmed by experiment before the limits of their 

 validity can be recognised. The latter, that is to say the 

 conditions which must be fulfilled for complete validity, are 

 often discovered much later, as our experience grows. Up 

 to tlie present, the only principle possessing unlimited 

 validity appears to be the energy principle of Robert Mayer. 



In his experiments on electric oscillations and waves, 

 Hertz had continually to observe small electric sparks pro- 

 duced by the resonators. In this connection he noticed that 

 the sparks were longer when the light of another simultan- 

 eous spark, or any kind of ultra-violet light, fell upon the 

 spark gap. He also discovered that it was chiefly or exclu- 

 sively the negative electrode upon which the effect de- 

 pended (1887). In this way he made a discovery, the further 

 prosecution of which has likewise opened up a large field of 

 knowledge: that of photo-electricity. As is always the 

 case, the main matter was to produce the new phenomenon 

 in the simplest possible form, which was very soon done 

 (1888),^ and then to follow it up with experiments of the 



^ Hallwach's effect. 



