RADIATION AND ODOURS 199 



sufficient to give a definite and discriminative sensation 

 through the organ of smell is of a minuteness beyond all cal- 

 culation or conception. These two facts — the almost infinite 

 individual diversity of smell and the almost infinite minute- 

 ness of the particles exciting it — render it very difficult to 

 form a satisfactory conclusion as to the nature of those 

 particles. It has been from time to time suggested that the 

 end-organs of the olfactory nerves may be excited, not by 

 chemically active particles, but by " rays," olfactive undula- 

 tions comparable to those of light. Physicists have not yet 

 been able to deal with the problem, but the recent discoveries 

 and theories as to radio-active bodies such as radium may 

 possibly lead to some more plausible theory as to the 

 diffusion and minuteness of odorous particles than any 

 which has yet been formulated. An example of the 

 minuteness of odoriferous particles is afforded by a piece 

 of musk which for ten years in succession has given off 

 into the changing air of an ordinary room " particles " 

 causing a readily recognised smell of musk, and yet is 

 found at the end of that time to have lost no weight, that 

 is to say, no weight which can be appreciated by the finest 

 chemical balance. An analogy (I say only an analogy, 

 a resemblance) to this is furnished by a pinch of the salt 

 known as radium chloride, no bigger than a rape-seed, and 

 enclosed in a glass tube, which will continue for months 

 and years to emit penetrating particles producing continu- 

 ously without cessation most obvious luminous and 

 electrical effects upon distant objects, the particles being 

 so minute that no loss of-weight can be detected in the 

 pinch of salt from which they are given off. 

 The sense of smell is of service to animals — 

 (i) In avoiding enemies and noxious things. 



(2) In tracing and following and di*riminating prey or 

 other food. 



(3) In recognising members of their own species and 



