"No element," says Frankland, " either alone or in combination, can exist with any of its bonds disconnected; hence the molecules of all elements with an odd number of bonds are generally diatomic, and always polyatomic—i.e., they contain two or more atoms of the element united together. Thus: Itrmacalcb VIrephin en, Q 0 Cialarlao, crg 0 0 Nitrogen, afd Off.0 Phosphor, P ISO 0-0 An element with an even number of bonds can exist as a monatomic molecule, its own bonds satisfying each other. Thus: embotto GraplIch Mercury, (F) Ws, Zab This graphic notation is most useful in fixing upon the mind the true meaning of symbolic formula;, and in elucidating the internal arrangement of the very complex molecules which often occur in both mineral and organic compounds. It also affords an easy means of showing the causes of isomerism in organic bodies. The following example will suffice to illustrate our meaning. The simplest of the alcohol family, methylic alcohol, is derived from marsh-gas by the substitution of one atom of Frank land's hydroxyl, Ho or HO (0 = 16), for one of hydrogen.
C114 C113110 (or C140).
0 0 000 0000 0 Harsh-gas. htethylIc alcohol.
27u classification of organic compounds has, during the last few years, been much improved. Until a comparatively few years ago, organic compounds were arranged, according to their most obvious properties, into acids, bases,fatty bodies, etc. Now the great majority of these compounds are arranged in series, of which each group dif fers from the preceding one by a fixed additional number of certain atoms. Thus (see Armstrong's Organic Chemistry, pp. 143, 144) twelve alcohols are represented by the gen eral formula (new notation), the first being represented by the others differing from it by an additional number of multiples of CIL. Bodies of analogous prop erties thus united are termed homologous. Again, every compound in a homologous series yields other compounds differing in composition from that from which they are derived, but yet bearing a different relation to it. Thus, alcohol yields ether, aldehyde, and acetic acid, and these so-called keterologous bodies form collateral series. This mode of classification is daily extending. It includes the organic radicals, such as methyl, ethyl, allyl, phenyl, cyanogen, etc.; the hydrides of the compound radicals, such as methylic hydride or marsh-gas, benzol, cyanic hydride or hydrocyanic acid, etc.; the alcohols, which form one of the most important of the families of organic compounds, and which are considered in a special article in this ENCYCLOPAEDIA; the aldehydes and ethers, both of which are specially described; the acids, of which the monobasic acids alone include six series, amongst which are the acetic or fatty series, represented by the general for mula and containing 19 or 20 distinct acids, the oleic series, the lactic series, the benzoic or aromatic series, etc.—while the dibasic acids may be divided into four series, in which occur the succinic series, containing nine acids, most of which present several modifications, and the tartaric series; the anhydrides (q.v.), of which those be longing to the acetic acid group may be arranged in series; the ketones or acetones; the compounds of nitrogen containing the amines, amides, imides, etc.; and, in short, except ing the natural alkaloids, the protein-compounds and their derivatives, the uric acid group, pigments, etc., there are few organic compounds which will not soon find a definite place in a series.
In this article we have strictly confined our remarks to the subjects bearing on gen eral, and for the most part on theoretical chemistry. We may, however, allude, in con clusion, to two subjects, which have undergone a great development during the last few rolumetric analysis and the synthesis of organic bodies, both of which are dis cussed in special articles.
The general tenor of this article shows that chemistry is at present in altogether a transitional state. As prof. Anderson of Glasgow observes in his address to the chemi cal section of the British association in Sept., 1867, the atomic theory, which, at the commencement of the present century, sufficed to explain all the facts of chemistry that were then known, is now quite inadequate to that end. At that time, chemists were acquainted with comparatively few compounds, and in these, oxygen was of such preponderating importance, that the science might have been almost termed " the chem istry of oxygen." Oxygen is now deposed from its high place, and is supplanted by
carbon to such a degree, that one of the first living chemists has actually proposed for organic chemistry the name of " the science of the carbon compounds." Facts gradually accumulated in the course of time which did not admit of explanation on the Daltonian theory; and as their number increased, such terms as catalysis, allotropy, etc., were invented, under which such facts were grouped together as were supposed to depend on similar causes. Such grouping may have certain temporary advantages, provided it is understood that, to use prof. Anderson's words, it is "the grouping of ignorance." It is indeed obvious that a true theory of chemistry must be a part of a general theory of dynamics, and that until we obtain some more distinct idea of how the atoms are grouped in the molecules of substances (see ATOMIC THEORY) than we at present possess, the link connecting theoretical chemistry and theoretical dynamics is wanting. The doctrine of atomicity evidently points to some general truth; it has been of great use in grouping together numerous facts, and in leading to investigations which have resulted in the discovery of many new facts and new generalizations, but we now want an explana tion of this doctrine, and this chemistry does not appear to be able to give us. The want of a theoretical explanation does not, however, render a generalization valueless, and much progress has been made of late years in ascertaining the " chemical structure " of substances—that is, in obtaining graphic formula, which consistently represent all the reactions by which the substances are formed or transformed. Before discussing the subject of chemical structure, it will be well to consider somewhat more fully than has been done above, the reasons why certain numbers have been selected for the atomic weights of the elements rather than any multiples or submultiplcs of them (see ATOMIC WEIGHTS). It was pointed out by Dulong and Petit that a close relation exists between the specific heat of a solid elementary substance and its atomic weight. Thus, if we take the old system of atomic weights (q.v.), and multiply the specific heat of each solid element by its atomic weight, we find that the elements form three groups. In the first, the product of specific heat into atomic weight, or atomic heat, varies from 6 to 6.6. In the second it varies from 3 to 3.3. In the third group, containing the allied elements, carbon. boron, and silicon, no regularity can be traced. By far the greater number of solid elements belong to the first or second group. Now it is plain that the atomic heat of a member of the first group is approximately double that of a member of the second group. But as the atomic weights are to a certain extent arbitrary, we can make the atomic heats of the two groups agree US- doubling the atomic weights of the members of the second group. This was first proposed by the eminent Italian chemist, Cann izzaro, and has now been accepted by most chemists. These new atomic weights not only greatly simplify Dulong and Petit's law, but are also in harmony with many other facts, most of which were observed after the change had been made. Thus the formula; of corrosive sublimate, bichloride of tin, and zinc methyl are, according to the old system, HgCl; SnC12; and and H = 1. According to the new system, they are and It will be at once observed that the second set of formula; represent just twice the quantity represented by the first; now the second formula express the molecular weights of the substances according to Avogadro's law (see ATOMIC THEORY). Further, if we adopt the old atomic weights, we see no reason why oxide of lead should readily form basic salts, while oxide of silver does not. This peculiarity is to some extent explained by the new atomic weights; thus we have nitrate of silver—old formula new formula AgNO2; nitrate of lead—old formula new formula Pb(NO2)2; basic nitrate of lead—old formula new formula Ph20(NO2)2. The contrast will be better seen if we put the new formulae into a graphic form.