34 AN INTRODUCTION TO SCIENCE 



of another, still less can it resolve a combination into its 

 constituent odours. How different it must be in the dog, 

 which can trace its master's footsteps out of a thousand, or 

 follow them even when the master, to hide his trail, puts oil 

 of bergamot on his boot-soles. By the time middle life is 

 reached, even the small portion of our brain which is 

 allocated to the sense of smell shows atrophic degeneration, 

 proving that the sense is disappearing as we might discover 

 by careful observation ; although as a general rule, from 

 force of habit and because we hardly ever call it into action, 

 we suppose that we still retain it. Its ever fading pictures 

 still delight or shock us. 



Stimulation of the olfactory membrane gives a ' ' massive 

 sensation ;" it is not marked by detail, that is to say. It is 

 for this reason that scents (and the same is true in a less 

 degree of tastes) recall scenes in a way which other sensa- 

 tions cannot do. The syringa which surrounded the 

 summer-house in which we played as children, the jasmine 

 beneath our bed-room window, the smell of warm pepper 

 with which a particular sausage-factory reeked we never 

 smell syringa, jasmine, pepper, without recalling these 

 vividly toned scenes. Anything seen with the eye or heard 

 with the ear would have characters of its own, but the scent 

 of syringa is the same whenever and wherever we smell it, 

 and it must always be the background to the first strongly 

 associated visual picture. 



The olfactory membrane responds to the particles of 

 certain chemical substances which have a comparatively 

 rapid proper vibration, especially such substances as the 

 essential oils. It cannot answer to a gas, of which the 

 atomic weight is less than 15, nor to bodies of considerable 

 atomic weight, such as the salts of the heavier metals. 



Sight. This is the sense upon which Man chiefly 

 depends ; and there is no reason to think that his eye is, in 

 its range of distance from objects near at hand to objects on 

 the horizon, its power of distinguishing detail, or the 



