The chemical nature of a carbon compound depends on the arrangement of its constituent atoms, but in its physical characters on the number of the atoms.

The physical character of a compound greatly influences its physiological action, a gaseous body being more easily absorbed and excreted than a liquid, and a liquid more easily than a solid. There will also be differences amongst the gaseous, liquid, and solid bodies themselves, for if a liquid, for example, has a low boiling-point so as to volatilise readily at ordinary temperatures, it will more resemble a gas in its action, while a liquid which has a high boiling-point will act more like a solid.

Thus in the group of hydrocarbons belonging to the paraffin .series the lowest members are gaseous at ordinary temperatures, the highest members form solid wax-like bodies, while those which are intermediate are liquid. Obviously we cannot expect a gas which can be inhaled in large quantities, and which will be quickly excreted when pure air is inhaled instead, to have the same action as a solid wax-like substance which can only be slowly absorbed, and slowly excreted or broken up in the organism.

The boiling-point of substances belonging to a series differs somewhat according to the chemical nature of the substance, but amongst the members of the series having the same chemical nature it rises with the number of atoms. It will be seen from the accompanying table that the boiling-point differs according to the series, e.g. that of chlorides is lower than that of bromides; this, again, is lower than that of iodides. In each series also of chlorides, bromides, or iodides, the boiling-point rises with the number of carbon atoms which the member of the series contains.

As the action of substances depends so much on their volatility, it may be convenient to give here the boiling-points of the various members of the paraffin series (p. 762).

Physiological Action of Hydrocarbons belonging to the Marsh-Gas Series. - These hydrocarbons may be regarded as hydrides of the radicals, methyl, etc. Those low in the series - methane, ethane, propane, and butane - are permanent gases at ordinary temperatures, and when inhaled pure produce anaesthesia much like that of nitrous oxide. The intermediate fluid members of the series, e.g. pentane, and substances containing them, as benzin, petroleum oil, etc, give off vapour having an anaesthetic action somewhat like that of chloroform.

The higher members of the series are solid at ordinary temperatures, and are used as a basis for ointments under the names of petrolatum, vaseline, cosmoline, etc.