While the number of official preparations made from organic substances is small at present compared with the many that have gained medical favor, and while those accepted by the U. S. Pharmacopoem are most important and universally employed, yet it has been deemed wise to include here additionally a few that are scarcely secondary, and which may have a future recognition. So many, built artificially and synthetically, are but reproductions in strength, effect, medicinal properties, etc., of natural plant-products, that we reasonably may expect a time when many such will replace, to a great extent, those from all other sources., As materia medica products are either inorganic or organic, and as the former comprise two great series: metals and metalloids, so the latter has two distinctive series: fatty and aromatic, consequently in this relationship-sequence they will be considered.

For a long time the term organic medicines was applied only to those agents taken directly from plants or animals - from a source built up of manifold varying organs in which alone was supposed to reside "vital force," by which all substances only could be created. We no longer hold tacitly to such restriction, because these very substances often are reproduced by a direct combination of the component elements or from apparent inorganic material which once was organic in nature.

Organic carbon compounds must contain, however, carbon and hydrogen, also frequently oxygen, nitrogen, sulphur. Such elements so differ from each other in chemical and physical properties, while their combining quantities are so unrestricted, that they unite in varying proportions to yield many hundred substances, each differing somewhat from the other.

Carbon, being quadrivalent, is considered the graphical centre around which the other elements are arranged - linked as it were by a bond dependent, as to strength and degree, upon each element's atomic valence. The grouping of these carbon atoms in a molecule is quite different in the two organic series.

I. Fatty Series: Derivatives of methane, CH4, and are linked so as to form an open chain in which the end carbon atoms are not equivalent to the others:

Methane, CH4

Ethane, C2H6

Propane, C3H3

Butane, C4H10

II. Aromatic Series: Derivatives of benzene (benzol), C6H6, and are linked to form a close chain in which every carbon atom is the equivalent of each other. This chain (skeleton) has been assumed from the fact that as soon as 6 or more carbon atoms come together they tend to so unite as to form stable and permanent compounds into which only definite amounts of other elements can be introduced.

Benzene, C6H6

Thus benzene takes up 2-4-6 atoms of hydrogen, chlorine, bromine, and under the prolonged action of HI becomes C6H12, and no more II can be imparted even if the action be continued indefinitely, whereas if carbon atoms were in open chain we would get C6H14, etc. Then, again, when the hexa-chloride or bromide of benzene is formed, each refuses to take up any more of either element, thus showing roundness or entireness in saturation and composition.