ON SMELL. 
289 
smell. But all persons are not able to smell it. I have remarked 
an average of one in every five persons who are totally unable to 
detect its odour. Here we reach the lowest limit of molecular 
weight. To produce the sensation of smell, then, a substance 
must have a molecular iveight at least 1 5 times that of hydrogen. 
If we compare the hydrocarbons of the paraffin series with each 
other, and similarly the olefine series, we notice that the lower 
members have no smell. The specific gravity of marsh gas, CH 4 , 
is 8 ; that of ethane, 15; propane is 22 times as heavy as 
hydrogen, and here we first notice smell. Olefiant gas has the 
specific gravity 14, and has no smell ; propene has a faint smell 
with a specific gravity of 21 , and the higher members of the 
series increase in intensity of smell with increase in specific 
gravity. Hydrocyanic acid is smelt by most persons, but not 
by all. Its specific gravity is 15. The higher members of the 
series, called the nitriles, have all very characteristic smell. 
Formic acid vapour has the specific gravity 23, and has a 
purely pungent odour. 
Acetic acid, 30 times as heavy as hydrogen, has a faint 
smell when pure ; propionic, butyric, and valerianic acids have 
strong smells. Methyl alcohol has no smell, its specific gravity 
is 16. Ethyl alcohol, 23 times heavier than hydrogen, has a 
faint smell ; and, as usual, the intensity, and if I may so term 
it, the flavour of the smell increases as we rise in series. 
These are the most typical instances of the carbon compounds, 
and they suffice, I think, to show the justice of the assertion 
that the intensity of smell increases with rise of molecular 
weight. The hypothesis of vibration satisfactorily explains this. 
The period of vibration of the lighter molecules is too rapid to 
affect our sense; there is a limit to this power; and just as 
some people have the power of hearing more acute sounds than 
others, so some noses are limited by a specific gravity of 15, and 
cannot smell prussic acid. Such people also have difficulty in 
