ON SMELL. 
291 
And it is this which appears to me probable ; so probable, 
indeed, as to form a working theory. 
But it is to radiant heat, I think, that we must look for 
indications of harmonics of the fundamental vibrations, which 
are, according to this theory, the cause of smell. And a fresh 
proof may be drawn from the indications already seen. Pro- 
fessor Tyndall has shown the power which odours have of 
absorbing heat rays. There is no doubt that hy refracting such 
heat rays by means of a rock-salt prism, after they have passed 
through an atmosphere of odour, certain portions of the heat 
spectrum show colder spaces, each corresponding to one parti- 
cular rate of vibration, which is absorbed by the vapour, through 
which the heat rays have passed. By measuring the position of 
such gaps in the heat spectrum, calculating the particular rate 
of vibration of the rays at such gaps, and referring them to 
their fundamental, we should arrive at the rate of vibration of 
the molecule which causes smell. 
We may now" enquire what it is which produces quality of 
smell. This, I think, can also be explained by the vibration 
theory, and depends on the harmonics of the vibration. Thus, 
the quality of tone of a violin differs from that of a flute by the 
different harmonics or overtones peculiar to each instrument. 
I would ascribe to harmonics the quality of smell possessed 
by different substances. And it is to this that compounds of 
chlorine, phosphorus, &c., owe their peculiarity of odour. The 
odour of compounds resembles that of their elements to some 
extent ; this may be accounted for by the similarity of overtones 
of compounds and their elements. 
Then we notice a similarity in quality of the odour of the 
compounds of a series like the alcohols, and yet the quality 
grows flatter and heavier with increase in molecular weight. 
Smell, then, may resemble sound in having its quality 
influenced by harmonics. And just as a piccolo has the same 
