ORGANIC.] CHEMISTRY 551 This list might be considerably extended, but sufficient examples have been given to illustrate the phenomena under consideration. Not only do these isomerides differ in physical pro perties, but they exhibit different and characteristic trans formations under the action of the same reagent. Thus, normal propyl alcohol when oxidized yields propionic acid : H H H H H H C C C H + 2 = H C C C H + OH 2 III II H H H - H H Normal propyl alcohol. Propionic acid. Water. Under the same circumstances pseudopropyl alcohol yields the substance known as acetone or dimethyl ketone : H H H C H H C H I I H C 0-H + O= C=0 H C H H C H Pseudopropyl alcohol. H Acetone. OH Water. The distinguishing character of this class of isomers is that the isomerides can be shown to belong to the same series of compounds, or, according to Schorlemmer, " they contain the same number of carbon atoms linked together." Reactions that give rise to the formation of a compound capable of existing in two isomeric modifications frequently result in the production of both isomers, but the precise conditions which regulate the relative quantities of the two compounds are not yet known. Certain relationships have been shown to exist between the physical and chemical properties of isomeric bodies, although numerous exceptions render the exact expression of these relationships an impossibility in the present state of knowledge. Thus, as a rule, the boiling points of the compounds of an isomeric series are lower than those of the normal series (see preceding table) or, more generally, the boiling point is higher the more simple the constitution of the substance. Since different amounts of heat are concerned in the production of isomeric bodies, it must be admitted that such bodies are stored with different amounts of potential energy. Adopting this view, some chemists have recently sought an explanation of isomerism in the different amounts of potential energy thus contained in isomerides, and have thrown distrust upon the "constitutional" theory. In taking this view, however, the true position seems reversed the fact that isomerides contain different stores of potential energy by no means does away with the hypo thesis that they possess different constitutions. It seems, on the contrary, that the difference of energy is accounted for on the view that the bodies possess a difference of constitution, since the contained energy results from the relative positions of the atoms or radicles with regard to the intra-molecular chemical forces. Metamerism. Compounds having the same molecular formula may result from the combination of totally distinct radicles, and exhibit in consequence not only a marked difference of physical properties, but in almost all cases different chemical transformations under the influence of the same reagent. Such substances are said to be meta- meric. The subjoined examples illustrate this class of cases. It is to be observed that, as with true isomeric compounds, two metameric bodies are frequently produced in the same reaction. For instance, most of the methods given for obtaining ethyl cyanide yield a mixture of this compound with the isocyanide. Empirical Formula, C 3 H 9 N . C 3 H 7 CH 3 CH 3 N" N N / H H Propylamine. Methyl-ethylamine. Trimethylamine. Empirical Formula, C 3 H 6 O . H H H 5 C 3 H H C C C II II 3 C C CH 3 N / C 2 H 5 H Allyl alcohol. H H Propionic aldehyde. Acetone. Empirical Formula, C 3 H 5 N . H 5 C 2 C=N Ethyf cyanide or propionitrile. H 5 C 2 N= Ethyl isocyanide or ethyl carbamine. Polymerism. Compounds having the same percentage composition but different vapour-densities are said to be polymeric ; thus Name of Compound. Formula. Vapour- density. Boiling- point. Acetylene C 2 H 2 13 Gas Benzene CH -3(0,11.,) 39 81 Sty rolene C 8 H -4(C,,H.,) 52 145 Dihydronaphthalene .... C 10 H 10 = 5(C s H a ) 65 210 The above list exhibits the rise in boiling-point with in crease of vapour-density. Polymeric bodies may be isomeric or metameric ; the following, for example, are metameric polymerides : H COH CH 3 COOH Formic aldehyde Acetic acid .., Lactio arid C 2 H 4 OH tactic acid COOH CLASSIFICATION OF ORGANIC COMPOUNDS. Homologous Series. Carbon being a tetrad element is only saturated by four atoms of a monad element, or by any number of atoms the joint atomicities of which are equal to four. Thus the following are saturated compounds : . CH, , cHcy , co"ci 2 , co 2 " , CN "cr , ci 4 . Among inorganic compounds many instances occur in. which two or more atoms of the same element unite, such, for example, as in the ferric and manganic salts. In these cases, however, the number of atoms thus entering into combination is seldom great. In the carbon atom we meet with the greatest tendency to unite with similar atoms, and this special property of carbon accounts for the great multiplicity of organic compounds. Making use of graphic notation, the tetrad carbon atom is thus represented C . If two atoms of carbon unite by one bond of each, the resultant atomicity of the group is 6; if 3 atoms unite the resultant atomicity is 8, and so on : _C_ C C C -c- -c- I ! c c 1 -A-
Thus every additional atom of carbon brings two active