GRAPHICAL FORMULAS. The theoretical as sumption just referred to is ( I) that an atom of carbon is quadrivalent. i.e. has four times the combining capacity of an atom of hydrogen, and, therefore, can hold in combination four 'univa lent' atoms like those of hydrogen and chlorine, or two 'divalent' atoms like those of oxygen; (2) that two or more carbon atoms may be directly combined with one another and may thus partly satisfy one another's combining capacity. This assumption, together with a knowledge of the valeneies (combining capacities) of other elements, makes it easy to determine a priori the several different combinations that are pos sible with a given set of atoms, each arrange ment being represented by a scheme, called a graphical formula, in which the atoms are repre sented by the chemical symbols of the elements, and their combining capacities by dashes that link together the symbols. The set of atoms CH, can be represented, on the above assumption, by only one graphical formula, viz. • Similarly, the set can be represented by thryc different graphical formulas; the set by lire formulas, etc.
When a compound is discovered whose moleen lar formula can be represented by only one such graphical scheme, the ease is simple, and the structure of the molecule becomes known at once. Further, in such eases the inference from the theory is that only one compound of the given molecular formula is capable of existence. In this manner, for instance, the theory gives us the constitution of marsh-gas, CIL, of ethane, (a constituent of eoal-gas), of propane, etc.; the verdict of the theory being, further. that only one compound CII„ only one compound CA, and only one compound are callable of existing. The fact that the most careful researches have actually led chemists to the discovery of one, and only one, compound cor responding to each of these molecular formulas, speaks strongly in favor of the structural theory. Again, in eases in which more than one graph ical formula can be constructed from a given set of atoms, the number of compounds actually known is generally the same as the number of formulas. Thus, we have seen that two
different structural formulas correspond to the molecular formula and, as a matter of fact, two, and only two, compounds of the formula can be obtained, viz. the sub stances known, respectively, as butane and iso butane, which have the same composition and the same molecular weight, yet differ considerably in their physical and chemical properties. Similar ly, three different compounds of the formula are known, five different compounds of the formula etc. In cases in which the num ber of isomeric compounds actually known was less than the number indicated as possible by the structural theory, earnest research has finally led to the discovery of the wanting isomerides.
The correspondence between the chemical prop erties of a compound and the relations exhibited by its graphical formula is usually capable of experimental demonstration. In the case of marsh-gas the graphical formula is symmetrical, because the doctrine of vaieney assumes no differ ence between the- several valeneies of an atom. We may, accordingly, expect that the four por tions of hydrogen contained in the compound. exercise precisely the same function and are in all respects identical. If there were two dif ferent positions in the graphical formula, which we will call positions A and B, then we could obtain, for example, one derivative by substi tuting chlorine for hydrogen in position A while leaving hydrogen in position B: and we could obtain a different derivative by substituting chlorine in position B while leaving hydrogen in A. In other words, two different mono chloro-substitution products of marsh-gas would he possible. But the mono - chloro - substitution product has been obtained by many different methods, and the product was always found the same. All efforts to produce two different deriva tions have failed. The conclusion is that the several portions of hydrogen contained in marsh gas have really the same function.