686 THE POPULAR SCIENCE MONTHLY. 



Numerous classifications of odors have been proposed. It is, 

 of course, impossible to quote any rational classification. The 

 natural way is to group around a type, in successive series, odors 

 which resemble one another. Eugene Rimmer has tried to do 

 this in the accompanying table. 



The author observes that it would be hard to arrange in any 

 of these series certain peculiar odors like that of wintergreen, or 

 salicylate of methyl and magnolia. Notwithstanding the uncer- 

 tainties attending the arrangement, we must apparently depend 

 upon classifications based upon this principle for a guide in the 

 study of odors. 



All that we know concerning the propagation of an odor is 

 that it consists in an emission of solid, liquid, or gaseous particles. 

 This emission is allied for these three states of matter to the 

 property called diffusion, which consists in the reciprocal pene- 

 tration at the end of a certain time of the particles of two or 

 more bodies among one another ; and also for solids and liquids 

 to the property called volatility, or the rapidity of evaporation. 



But little is known concerning the diffusion of solids. If we 

 heat to a high temperature a porcelain crucible within a crucible 

 of plumbago, the plumbago will penetrate the porcelain to a 

 depth varying according to the duration of the experiment. M. 

 Pellat has shown, by delicate measures of quantities of electricity, 

 that metallic surfaces placed parallel to one another a few tenths 

 of a millimetre apart, reciprocally exchange their outer surfaces, 

 as if they emitted a little of their own substance to each other. 

 When the influence ceases, the surfaces gradually lose their 

 foreign coatings, and return slowly to their primary condition. 



The diffusion of liquids is easily observed. It can be wit- 

 nessed by introducing, with a pipette, into a vessel under water 

 a colored liquid, red wine, for example. The wine, being lighter 

 than water, rises to the surface, and does not color the deeper 

 layers of the water till after one or two days. There is doubtless 

 in the complicated diffusion of liquids a kind of chemical action 

 related to the movements on water of camphor and a consider- 

 able number of diffusible substances. If we put a bit of camphor 

 on the surface of water, it at once turns round and moves in 

 every direction. If a drop of oil is let fall on the same surface, 

 the movements will cease immediately. The motion arises from 

 the diffusion of camphor in a liquid form on the surface of water. 

 When, after the surface is saturated, there is no more diffusion, 

 the motions cease. They also cease when two currents are pro- 

 duced by different bodies in opposite directions. That there is a 

 liquid diffusion is proved by the fact that when the camphor is 

 placed on a float of pith, or on the polished surface of mercury, 

 there is no movement. So, if a bit of camphor is put into a large 



