Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/556

Rh 544 CHEMISTRY [ORGANIC. (?) NClj PC1 OC1. SC1 4 ? Cl affords evidence confirmatory of the assumption that hy drogen has metallic properties. We leave it to the reader to trace out the alteration in properties which accompanies alteration in atomic weight; but we may point out as an instance that the power to combine with other elements is subject to periodic varia tion ; thus, the composition of the highest chlorides and oxides of the elements in the first and second horizontal series in the above table is as follows : Chlorides. LiCl GC1, BC1, CC1 4 NaCl MgCI, AlgClg SiCl 4 Oxides. Li 2 GO B 2 O 3 C0 2 N 2 5 3 ? Na 2 2 MgO A1 2 3 Si0 2 P 2 5 S0 3 C1 2 3 The chlorides and oxides of the elements of each of the following horizontal series exhibit similar differences in composition. With very few exceptions, only the ele ments which are included in the same vertical series as carbon, nitrogen, oxygen, and fluorine form compounds with hydrogen, the stability of which diminishes with increase of atomic weight; but the composition of these hydrides is subject to periodic variation, as is shown by the following list : CH 4 NH 3 OH, FII SiH 4 PH 3 SII, C1H AsH 3 SeH, BrH SbH 3 Tell, III The periodic character of the relation between the proper ties of the elements and their atomic weights is especially evident in their physical properties, which are more readily compared than the so-called chemical properties, as they admit of measurement, although, on account of our imper fect knowledge, comparison is at present possible only to a very limited extent. One of the few physical properties which has been determined for most of the elements is the specific gravity in the solid state ; now, on comparing the specific gravities of the various elements we find that they do not increase progressively with increase of atomic weight, but that they increase and diminish periodically. Hence, as Lothar Meyer has shown, on taking the atomic volumes, or quotients of the atomic weights of the elements divided by their specific gravities in the solid state, as abscissae, and the atomic weights as ordinates, a curve is obtained which exhibits a series of maxima and minima, viz., five maxima and five minima in the portion which includes barium and the elements of lower atomic weight. The most electro-positive elements, lithium, sodium, potas sium, rubidium, and caesium form the five maxima ; but with increase of atomic weight the height to which the curve rises also rapidly increases, namely, in the proportion Li : Na : K : Rb : Cs = 1 2 : 24 : 46 : 57 : 79. The remaining physical properties of the elements, so far as they are known to us, appear to be subject to similar periodic variation, but for a full discussion of the connec tion between the atomic weights of the elements and their properties the reader is referred to L. Meyer s work, Die modernen Theorien der Chemie (Breslau). The establishment of the periodic law may truly be said to mark an era in chemical science, and we may anticipate that its application and extension will be fraught with ths most important consequences. It reminds us how im portant above all things is the correct determination of the fundamental constants of our science the atomic weights of the elements, about which in many cases great uncertainty prevails ; it is much to be desired that this may not long remain the case. It also affords the strongest encouragement to the chemist to persevere in the search for new elements. (H. E&amp;lt; A _) ORGANIC CHEMISTRY. As has been already explained (p. 520) it was at one time thought that certain chemical compounds were pro ducible only through the agency of living things, and the name &quot; organic chemistry&quot; was in consequence conferred upon this branch of the science. The progress of discovery has, however, served to break down the barrier that was supposed to exist between those and purely inorganic com pounds, since it has been found possible to build up artifi cially a very large number of compounds formerly regarded as essentially organic. Nor must it be forgotten that many living things elaborate compounds which can in no sense be regarded as organic such as the silica of grasses, the carbonate of calcium secreted by molluscs, &c., and the tri- calcic phosphate forming the bones of the higher animals. Although the number of elements entering into the composition of organic compounds is comparatively small, yet these compounds far exceed inorganic bodies both in number and complexity of composition. It is indeed the vast and ever-increasing numbers of the organic compounds that render it necessary to form a separate branch of the science for their study, and not any real chemical difference between the matter forming these and mineral compounds. Definition and Character of Organic Compounds. Carbon is an invariable constituent of organic bodies, so that this branch of the science is sometimes defined as &quot; the chemistry of the carbon compounds.&quot; The best chemical definition of organic compounds, however, is that proposed by Frankland, viz., &quot;compounds the molecules of which consist of one or more atoms of carbon directly combined either with carbon, nitrogen, or hydrogen.&quot; After carbon, the elements of most frequent occurrence in organic compounds are hydrogen, oxygen, and nitrogen. Sulphur, phosphorus, the halogens, silicon, boron, arid the metals are of comparatively rare occurrence. The number of atoms entering into the composition of organic molecules is often very great far exceeding the atoms of the most complex molecule of a mineral substance. Thus, stearin contains 173 atoms, and albumin no less than 226. Chemical bodies that are possessed of great complexity of composition are generally distinguished by the facility with which they split up under the influence of the physical forces and chemical reagents ; thus organic compounds are as a class characterized by their instability. Ultimate Analysis of Organic Compounds. Before the formula of an organic compound can be determined, it is necessary to ascertain the amounts of its several constituents that is to say, to make a quantitative analysis of it. The elements carbon and hydrogen being of paramount importance are generally first determined. The method employed depends upon the fact that all organic compounds undergo combustion when heated with easily reducible oxidizing substances, their carbon being oxidized to carbon dioxide and their hydrogen to water. These two products of combustion being collected in suit able apparatus and weighed, the necessary data for calcu lating the amounts of carbon and hydrogen are obtained. The following is a brief description of the process. Determination of Carbon and Hydrogen. A tube of Bohemian hard glass of about 10 or 14 millimetres internal Fia. ] .Combustion Tube. o-a, pure cupric oxide; a-b, mixture of substance and CuO; b-c, rinsings from mixing wire; c-J, pure CuO; d-e, metallic copper; e-f, plug of asbestos. diameter, after being scrupulously cleaned and dried, ia