Recent Advances in Electricity. 29 



summer evenings in Nova Scotia. Its flashes suddenly appear- 

 ing and as suddenly disappearing in mid-air are quite startling. 

 I caught one, and should describe it as a flying beetle or 

 cockchafer, the luminous part being under the wings, so as to 

 be sometimes covered and sometimes exposed. The appearance 

 which it presents when on active duty is very similar to the 

 sudden striking of a match. In these cases there seems to be 

 very little energy available for producing the observed lumi- 

 nosity, and the same remark applies to the phosphorescence of 

 several well-known salts of calcium and of strontium, which, 

 after being once exposed to the sun, remain self-luminous when 

 seen in a dark room for months or even years afterwards. 

 There is a shrewd suspicion in many minds that our present 

 methods, of producing artificial light are excessively wasteful 

 — that we produce an enormous quantity of heat, of which 

 only a small part is utilised as light. 



In connection with all our progress in the theoretical 

 knowledge of electricity and of the closely allied subject 

 of light, the question of the nature and constitution of 

 what we have been accustomed to call the luminiferous ether 

 is becoming increasingly prominent. We can no longer afford 

 to regard it as a mere working hypothesis, a fiction of the 

 imagination, or a thing of doubtful reality — a mere ghost ; for 

 we are giving it more and more work to do, and all our phil- 

 osophy breaks down without it. But it is in many ways a 

 puzzle. It allows all sorts of bodies to move through it without 

 any sign of resistance, and yet it serves for the propagation of 

 transverse vibrations — a property characteristic of a solid, as 

 distinguished from a liquid or a gas. The nearest approach to 

 it in ordinary substances is a jelly. A jelly possesses the right 

 kind of elastic properties ; but bodies cannot move through a 

 jelly without very considerable resistance, and it is not self- 

 healing when ruptured by their passage. 



Modern mathematicians would like to reduce all nature 

 to a frictionless, incompressible fluid, possessing certain in- 

 ternal motions — eddies, vortices, and so forth, which in 



