Let us take hydrogen as our standard of reference and consider some of its simplest compounds. In HCI, hydrochloric acid, one atom of hydrogen is united to one of chlorine. So also we have HBr, and HI, KCl and NaCI, AgBr, etc. These elementary atoms, which combine only in the ratio of one to one, at least to form permanent compounds, are univalent, that is, their power of fixing or uniting with other atoms is unity. In water, on the other hand, H2O, a single oxygen atom holds two of hydrogen in combination, and so oxygen is called a bivalent element. In H,S, CaCl2, we have illustrations of still other compounds in which a bivalent atom is united with two of the univalent type. Nitrogen, phosphorus, ar senic, aluminum and some other elements go still farther, and are trivalent, so that the com pounds formed by them have such formulae as NH,, PH., Asti., AIC1, and so on. Carbon, a quadrivalent substance, forms normally com pounds of still more complex type, such as CH., CCI., or, when it unites with two dyad or biv alent atoms, CO. and CS,. For brevity the valency of the various elementary atoms may be indicated by speaking of them as monads, dyads, triads, tetrads, etc. The rare metal vanadium is a pentad, having a valency of five, and sexivalent tungsten is a hexad. In many cases valency seems to be a variable property of the atom, as, for example, when we consider the two chlorides of phosphorus, Pas and PO.. In cases like these the higher figure may be taken as showing the maximum atom-fix ing power of the element, a power which is only partially exercised in the lower com pounds.
In the theory as thus stated there is no mere speculation; it is a statement of definitely observed facts. It tells us that the atoms unite, not arbitrarily, but in accordance with certain rules; and these help us in our comprehension of known compounds and the discovery of new ones. Indeed, an enormous advance in chemi cal discovery followed the application of these principles: an advance not only of scientific importance, but of economic and commercial value also. It was in the domain of organic chemistry that the growth became most imme diately evident, and to this branch of the sci ence we may now turn our attention. It is here that we find the best illustrations of what is meant by chemical constitution or structure, and the best examples of isomerism.
— Its already been pointed out that organic chemistry in its beginnings dealt with animal and vegetable substances, the products of liv ing organisms. Other artificial bodies, derived from these, were also included in its territory. All organic compounds were characterized by the presence in them of carbon, this element being ordinarily combined with hydrogen, oxy gen, nitrogen, or all three, and sometimes with other elements also. Organic chemistry, as its
domain was enlarged, in time received a new definition, and to-day the term broadly. signifies the chemistry of carbon compounds. It is true that some compounds, such as the metallic carbonates, are more conveniently described as inorganic in character; but these minor excep tions affect the definition but slightly.
At first sight the almost innumerable or ganic substances appear to be hopelessly com plex, and some, indeed, such as albumen, are so; but a closer inspection reveals order among them, and, in general, an ultimate simplicity. Their great number is due to the fundamental properties of the carbon atom, which, being quadrivalent,.can unite with four other atoms simultaneously; and which, moreover, may combine with other atoms of its own 'ChM to form rings or chains that serve as nuclei for the development of long series of substances. Most of the latter are derived from hydrocar bons, compounds of carbon and hydrogen, and these are exceedingly numerous. Marsh gas or methane, CH., contains the largest propor tion of hydrogen, and is the type upon which the quadrivalency of carbon is predicated; it is, furthermore, the first member of a series of hydrocarbons, CH., GIL, CiH., and so on up to G.H., and perhaps even farther. In this series each compound contains one atom of carbon and two of hydrogen more than the hydro carbon preceding it, and this regular difference establishes what is known as an homologous series. Nearly all organic substances can be arranged in series of this kind, so that the chemist is able to master a great number of de tails by a single effort of the memory. More over, the members of each homologous series vary regularly, step by step, in their physical properties. Thus CH, is a gas, C,H. a heavier gas, GH., a volatile liquid, the following terms are liquids which grow less and less volatile, while above GiHn the hydrocarbons are waxy solids. Ordinary paraffin is a mixture of these higher hydrocarbons, and the whole group, • therefore, is known as the paraffin series. Com mon petroleum consists chiefly of its liquid members. From these hydrocarbons, with oxy gen, a parallel series of alcohols is such as or wood alcohol, GH.0, • ethyl or ordinary alcohol, C.H.0, amyl alcohol or fusel oil, further oxidation the alcohols yield a set of acids, among which acetic acid, the acid of vinegar, is the one most farnittarly known. This single family of hydro carbons is the key to thousands of other stances which are directly producible from them. Some of these products illustrate very simply a mode of derivation which is known as substitution, as when, from CH., hydrogen atoms may be successively withdrawn and re placed by univalent atoms of chlorine. Thus see get the following compounds: CR., Clita,