ON SMELL. 
285 
already mentioned, some compounds of selenium and tellurium, 
tlie compounds of chlorine, bromine, and iodine with the above- 
named elements, and some metals. 
In considering this list, I submit, first, that the property of 
smell is peculiar to some elements and their compounds. Thus 
chlorine, bromine, iodine, sulphur, selenium, and tellurium, 
which are volatile, and give off vapour at ordinary temperatures, 
have a characteristic smell. We should expect the compounds 
to have a smell, and we find this to be the case. Second : those 
substances which have no smell, or produce simple irritation of 
the nostrils, have all low molecular weight. Such is the case 
with hydrogen, the element of lowest specific gravity. Such 
also is the case with oxygen and nitrogen ; but this, as well as 
the absence of smell in water, may be ascribed to the constant 
presence of these gases in our atmosphere and their necessary 
constant presence in our nostrils, so that we may be insensible 
to their smells because we are always inhaling them, but I think 
it probable that this is not so. Hydrochloric, hydrobromic, and 
hydriodic acids and ammonia have purely an irritating effect, and 
cannot be described as smells. When ammonia is pure and free 
from compounds containing carbon, it has no trace of smell. 
Nitrous oxide is also the lowest of the oxides of nitrogen, and as 
such has the lowest specific gravity. But it is when we turn to 
compounds of carbon, that we are best able to draw general 
conclusions ; for that element pm' excellence has the faculty of 
forming almost innumerable compounds, and series which 
resemble each other in properties, hut differ in specific gravity. 
And here we are most struck with the fact that increase of 
molecular weight, i. e., increase of specific gravity in the form of 
gas, produces, to a certain point, smell. Let us examine the 
simplest series, viz., the marsh.gas or methane series, commonly 
called the paraffins. 
The first two of these have no smell. Ethane, indeed, which 
