Carbon is a tetrad element. It is sometimes represented graphically thus:

It combines with four atoms of a monad, or two of a dyad element, e.g.

or

or it combines

with one atom of a triad and one of a monad, H -

It also unites with itself, and the complex molecules thus formed combine with other elements or radicals. Thus the number of its compounds is almost endless.

These compounds are divided into two great series, according to the mode in which the atoms are linked. •

We have thus in Inorganic Chemistry two great series, the metalloids and metals, and in Organic Chemistry two great series, the fatty and the aromatic.

Series of Carbon Compounds. - In the first, or fatty series, the carbon atoms are supposed to be linked so as to form an open chain, e.g.:

In the second, or aromatic series, the carbon atoms are supposed to be linked so as to form a closed chain.

Some of the simpler compounds of carbon have already been considered - carbonic acid, CO2 (p. 583), hydrocyanic acid, HON (p. 585), acetic acid, C2H4O2 (p. 577).

General Action. - It will be noticed that compounds of carbon with hydrogen alone, as in the hydrocarbons of the marsh-gas series; with oxygen alone, as in carbonic acid, CO2; with sulphur alone, as in bisulphide of carbon, CS2; or with chlorine alone, as in tetrachloride of carbon, CC14, all tend to paralyse the nervous system, and to destroy the functional activity of its various parts in a definite order. Thought fails first, next sensation, and next reflex action (p. 206).

The compounds with hydrogen have a comparatively slight action on muscle, but those containing chlorine are more powerful muscular poisons, and destroy the contractility of muscular fibre, both voluntary and involuntary.

Many compounds containing oxygen in addition to carbon and hydrogen have an anaesthetic action, e.g. alcohol and ether; others, like acetic acid, have a strongly irritant action. Compounds of carbon with nitrogen, hydrogen, and oxygen may have a very complicated chemical constitution, and, as in the organic alkaloids, have physiological actions which are too varied and specialised to allow of their being classed at present under a general law.