ON SMELL. 
293 
It is doubtful ^Yllether there exists a lower limit to our sense 
ef smell. The vapour of osmic acid is one of the heaviest known, 
and it has a most distinct smell. It is about 130 times as heavy 
as hydrogen. Tetrabromide of carbon is 166 times as heavy as 
hydrogen. The vapours of selenium, tellurium, arsenious oxide, 
and antimonious oxides are also extremely heavy. There appears 
to be a limit in practice, however, owing to the non-volatility of 
substances of high molecular weight, at such temperatures at 
which smell may be perceived. 
The intense perfume of flowers is to he ascribed to the 
terpenes, of which common turpentine is one, or to their products 
of oxidation, and these bodies all possess a molecular weight of 
136, and the specific gravity 68, a specific gravity which appears 
to excite the olfactory nerve most powerfully. 
I bring forward the theory adduced with great diffidence. 
The problem is to be solved, in my opinion, by a careful 
measurement of the lines in the spectrum of heat rays, and the 
calculation of their fundamentals, which this theory supposes to 
be the cause of smell. 
Such measurements and calculations, even if they proved 
the theory untenable, would have great value for their own sake, 
and labour expended in this direction would not be lost. Whether 
successful or not, it would at least be a first assault on what old 
John Bunyan called Nose-gate of the “ City of Maiisoul.” 
