SENSE OF SMELL. 181 



appear to have lost any of their odour. But these particles 

 cannot be as small as those of light, because we see that glass 

 is capable of retaining the former, but suffers the latter to 

 pass through it. The art of the perfumer consists in fixing 

 and preserving odours in the most agreeable and convenient 

 vehicles. 



8. Odours differ very much as to the permanence of the 

 impression which they produce. While that of some is very 

 transient, in others the scent remains for hours after the 

 application of the substance. They also differ, as we have 

 seen, as to the extent to which their influence extends. 

 .Moisture in the atmosphere is favourable to the diffusion of 

 odours, which would seem to show that vapour is a good 

 conductor. For example, a flower garden is never more 

 grateful to the smell than in the morning, when the dew is 

 evaporating, or after a warm summer shower. So also the 

 plants of a green-house are more fragrant, just after they 

 have been watered. Some flowers give off their odour only 

 at certain times, generally after they are fully expanded, and 

 their parts in the greatest activity. 



9. Though the air is the usual vehicle for odours, yet we 

 find that they adhere to solid bodies, and can even be con- 

 veyed through water. But though the whole art of per- 

 fumery is founded on this fact, it has been strenuously denied 

 that they could be conducted through water, and conse- 

 quently that fishes could smell. Some physiologists state 

 that fishes have no olfactory organ ; that the part commonly 

 considered as such is the organ of taste. This opinion is, 

 however, erroneous. " Not many naturalists of the present 

 day," says Dunglison, " will be hardy enough to deny, that 

 fishes have an organ, or sense of smell. At all events, few 

 anglers, who have used their oil of rhodium, or other attrac- 

 tive bait, will be disposed to give up the results of their 

 experience, without stronger grounds than any that have 

 been assigned by the advocates of that view of the subject." 



10. Fishes are furnished with organs of smell, but they 



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