230 CHEMICAL DISCOVERY AND INVENTION 



taste, depends on actual contact of the sensory surface with the 

 exciting substance, and it does not appear to depend like the 

 faculties of hearing and of sight on the entrance of impulses in 

 the nature of waves of air or of " ether " into the receiving ner- 

 vous surface. The extremely minute division of matters capable 

 of exciting smell may be inferred from common observations in 

 everyday life. One of the commonest experiences of a walk in 

 the country is the smell of burning weeds. When the vegetable 

 matter thus disposed of is heated it is partly consumed by the 

 aid of the oxygen of the air and is converted into vapours of 

 water and carbon dioxide which are inodorous. At the same 

 time a portion, but only a very small proportion, of the stuff is 

 by the heat alone, without combustion, made to yield sub- 

 stances such as acetic acid and phenols, which in a state of 

 vapour are acrid and disagreeable to the nose and eyes. They 

 are perceptible at great distances when diffused through the air. 

 They can be smelt when the accompanying smoke is no longer 

 visible and at a distance of half a mile or more. The matter 

 thus diffused would probably not exceed an ounce or two, but 

 it can be recognised when spread in this way through millions of 

 cubic yards of air. One is tempted almost to believe that under 

 such circumstances the sense may be awakened by separate 

 molecules arriving singly or a few at a time. 



The subdivision of matter in its extreme forms may, however, 

 be rendered perceptible by the eye if only the illumination is 

 sufficient. The passage of a sunbeam through a room in which 

 nothing could previously be perceived is enough to show that 

 the apparently clear air is filled with myriads of particles which 

 by reason of their small size remain suspended. These little 

 particles are, however, relatively monstrous, for they consist of 

 minute hairs, pollen, yeast cells, animal and vegetable debris 

 of all kinds, each of which possesses an organic structure, mixed 

 with tiny grains of sand or earthy matters blown up by the wind. 

 This process of illumination will help to reveal other effects in 

 which the actual masses of the particles can be calculated. If 

 one of the artificial colouring matters such as magenta, fluores- 

 cein, or eosine, of which the composition and molecular weight 

 are known, be dissolved in water the solution may be diluted till 

 it contains no more than 1 or 2 parts of the solid in 100 millions 

 of the liquid, and the colour will still be perceptible. When the 

 solution possesses the property of fluorescence which is beauti- 



