472 Professor Arthur Schuster [Feb. 22, 



( deductions, the normal fall of potential should, according to the 

 views of the author, have a different sign in the polar and equatorial 

 regions, which is contrary to the observed fact. This theory does 

 not, however, exhaust the possibility of explaining atmospheric 

 electricity as a phenomenon of electromagnetic induction, and it is 

 not disproved that in some form or other the rotation of the earth's 

 magnetic field may play a part in the origin of the electric field. The 

 theories which take solar radiation as the source of the energy divide 

 themselves into several groups. We may think of a direct thermo- 

 electric or actinic action, but there is, so far, no experimental support 

 to such views. One of the earliest and most natural suppositions is 

 the belief in evaporation as a source of electrification. This was 

 Volta's theory, and experiments have at various times been produced 

 in its support ; but, so far, no one has been able to invalidate 

 Faraday's conclusion that whenever electrification seemed to appear 

 as a consequence of evaporation, it was really due to secondary causes, 

 such as the friction of the liquid spray against the sides of the con- 

 taining vessel. Rejecting Volta's theory, there is nothing left but 

 the belief in some form of contact or frictional electricity either 

 between drops of water and air, or water and ice, or any two of the 

 various bodies present in the atmosphere. The possibility of contact 

 electricity between a solid or liquid and a gas, is not quite easy to 

 submit to the test of experiment. If we rub two solid bodies together, 

 we may, by separating them, investigate the electric field produced ; 

 but, supposing we have a drop of water surrounded on all sides by 

 air, the water may be covered with an electric layer of, say, positive 

 electricity, the air in contact with the water with the opposite kind, 

 and it is not at all clear how we could experimentally demonstrate 

 the difference of potential between the air and the drop which is thus 

 produced. A current of air flowing past the drop might carry away 

 some of the negative layer, and in this way an electric field may be 

 established while clouds are forming, but the conditions necessary for 

 an experimental demonstration would be very difficult to realise. 

 Two methods have been devised which practically demonstrate some 

 form of contact electricity between gases and water. 



Lenard, wishing to imitate the electric field observed in the 

 neighbourhood of waterfalls, has established by careful experiment a 

 number of important facts, which are all consistent with the following 

 explanation. If we imagine two oppositely electrified layers at the 

 surface of a drop of water such as has been referred to, and if the 

 drop falls on to a layer of the same liquid, or if similar drops impinge 

 on each other, the difference of potential produced by the fusion of 

 the surface layers becomes greater than is consistent with equilibrium. 

 For, taking the case of drops falling into a mass of water contained 

 in a cylindrical vessel, the extent of surface between air and water is 

 not increased by the falling drops, and we must imagine that surface 

 to be already covered with a sufficient electrical sheet to establish the 

 required difference of potential. The electrification of the drops is, 



