284 
ON SMELL. 
It is easy to prove that solid particles are not the cause of smelL 
If the air conveying the odour be filtered through a tube 
filled with cotton wool and inserted into the nose, a smell is still 
discernible, although all solid particles must thereby be kept 
back. But it is a very remarkable circumstance that it is so, 
for one would not suspect such extremely non-volatile substances 
as copper, iron, silver, &c., to give: off gas, if indeed the smell 
which they most certainly evolve, when rubbed, is due to the gas 
of the substance. 
We must, therefore, conclude that the sense of smell is 
excited by gases only. It is, of course, necessary to include 
under the name gases, the vapours of liquids or solids which 
have low vapour tension, and which, in consequence, give off 
vapour at the ordinary temperature. It has been proved that 
this is the case even with mercury, the boiling point of which is 
360° Centigrade. We may consequently conclude that many 
other substances, of which it is impossible to measure the 
vapour tension at ordinary temperatures, owing to its extreme 
minuteness, also evolve gas, if only in very small quantity. But 
it is well known that all gases have not the power of exciting a 
sense of smell. Let us compare some gases which have smell, 
with some which have none, and endeavour to discover if those 
which have smell have any other property in common. The 
following is a list of gases which have no smell: — Hydrogen, 
oxygen, nitrogen, water gas, marsh gas, olefiant gas, carbon mon- 
oxide, hydrochloric acid, formic acid vapour, nitrous oxide, and 
ammonia. Those which possess smell are — Chlorine, bromine, 
iodine, the compounds of the first two with oxygen and water, 
the second three oxides of nitrogen (or perhaps it is right to say 
nitric peroxide, for the other lower oxides are changed into it 
when they come in contact with air), the vapours of phosphorus 
and sulphur, arsenic and antimony, sulphurous acid, carbonic 
acid, and almost all the volatile compounds of carbon save those 
