286 
ON SMELL. 
is 1 5 times as heavy as hydrogen, begins to have a faint trace, 
hut it is not till we arrive at butane, which is 30 times heavier 
than hydrogen, that a distinct sensation of smell is noticed. In 
the same manner the olefine series, of which the first member is 
ethene, or olefiant gas gains in smell with rise of molecular 
weight. Of course the highest members of this series have no 
smell, for they are non-volatile, but this is the case with most 
carbon compounds of which the molecular weight is high. 
A similar relation is noticeable among the alcohols. Methyl 
alcohol in a state of purity is smell-less. Ethyl, or ordinary 
alcohol, when freed from ethers and as much as possible from 
water, has a faint smell, and the odour rapidly becomes more 
marked as we rise in series, till the limit of volatility is reached, 
and we arrive at solids with such a low vapour tension that they 
give off no appeciable amount of vapour at the ordinary 
temperature. Again with the acids. Formic acid is smell-less 
and produces a pure sensation of irritation. Acetic acid has a 
slight but characteristic smell ; and the higher acids of the 
series, propionic, butyric, valerianic acids, &c., gain in odour with 
increase in density in the form of gas. If we consider the 
nitrogenous compounds of carbon, wo are led to the same 
conclusion. Prussic acid is not smelt by more than four persons 
out of every five ; but the nitriles, which bear the same relation 
to prussic acid as the higher members of a series bear to the 
lower, have all very characteristic odours. Acetylene would 
appear to form an exception to this rule, but carefully-purified 
acetylene has little odour, and it is surpassed by its higher 
homolog ues. We may therefore, I think, accept this as a 
principle-— that the intensity of the smell rises with rise in 
molecular weight. 
It is also noticeable that the character of a smell is a 
property of the element or group which enters into the body 
producing the smell, and tends to make it generic. 
