246 OUTPOSTS OF THE INTELLIGENCE SERVICE 



a great extent to lielp him in the identification of compounds. 

 The tea blender and the wine expert can detect very shght differ- 

 ences in " flavour." 



It is worth while noticing that receptors all depend for stimula- 

 tion on the existence of an alteration in external energy. This 

 is specially marked in the case of this ancestral chemical sense. 

 Our accustomed environment presents no stimulus. Air has no 

 smell and water no taste. The introduction of a trace of foreign 

 body alters the energy content of the environment and stimulation 

 follows. It is a common experience to find that people do not 

 experience sensations which have, for the time being, become 

 permanent in their environment. A room may be stufl'y to an 

 incomer but quite comfortable to the tenants. The physiological 

 chemist works in an atmosphere which causes his visitor to choke 

 and splutter, but the introduction of a new odour, say ammonia, 

 is at once perceived and produces instant action. 



The primary odours are (i.) flowery (violet), (ii.) fruity (lemon), 

 (iii.) spicy (nutmeg), (iv.) resinous (frankincense), (v.) putrid (H2S), 

 and (vi.) burning (tar). Other olfactory sensations are mixtures 

 of two or more of these, e.g. vanilla = (i.) + (ii.), garlic = (ii.) + (v.), 

 and so on. 



How do these substances produce a stimulation of the olfactory 

 epithelium ? The present idea is that in the gaseous state they 

 produce a series of waves in ether, part of the electro-magnetic 

 spectrum, having frequencies much greater than those of light. 

 This is quite plausible, as odorous substances belong almost 

 exclusively to the fifth, sixth and seventh groups of the periodic 

 classification, in which the elements are characterised by the 

 possession of variable valences , i.e. can set free electrons. The 

 " strength " of a smell appears to be related to the speed of 

 rotation of the valence electrons (Chap. XIII.). It is interesting 

 to note that ultra-violet light, which is known to have the property 

 of stabilising these substances by destroying the double bonds in 

 them, also destroys their odour. 



8. Hunger is a sensation which must be regarded as primitive 

 and basal. It is not our business to analyse the feelings of hunger, 

 but to consider the mechanism bv which the lack of nourishment 

 is signalled to consciousness. The evolution of knowledge of this 

 sensation is largely due to Cannon, whose book on the subject 

 should be read by every student. There can be no doubt that the 

 feeling of hunger is closely allied to pain. 



" The sensation of hunger is difficult to describe, but almost 

 everyone from childhood has felt that dull ache or gnawing 

 referred to the lower mid-chest region or epigastrium, which takes 



