Page:The New International Encyclopædia 1st ed. v. 04.djvu/643

* CHEMISTKY. 561 CHEMISTRY. sulphur. In sepanitinjir the constituents of a niixluiv. advanta^ie is taken of the ditTerenccs in their properties — in the ease of hydrogen and oxygen, the ditTerenee in dilTiisibility : in the case of iron and sulphur, the faet that iron possesses magnetic properties while sulphur does not. The properties oftenesl taken advantage of for sepa- rating substances without destroying their in- dividuality are rolnlilili/ and soliibiUti/ : and on these are based, respectively, the processes of distillation and crystallization. When a sulKciently stable chemical compound is subjected to one of these pi'ocesses, and is thus gradually divided into two or more por- tions, the Latter are qualitatively identical with one another and with the whole. Thus, when jnire water is subjected to distillation, its com- position remains unchanged while any portion of it is being removed. The successive portions of the escaping va])or. too, must have the same composition — viz. that of i)ure water. Similarly, in the case of pure alcohol, when part of a given araoimt is removed by distillation, the portion remaining undistilled. as well as the distillate, cannot be anything but pure alcohol, and hence cannot but have the same composition as the liquid before the distillation. Quite different is the case of mixtures. T.et, for example, a liquid made up of alcohol and water be subjected to distillation, and let the process be discontinued when a portion of the liquid has passed over. The liquor remaining behind undistilled will then be found to be much weaker — i.e. to con- tain a greater projjortion of water — than the orig- inal liquor before the distillation. On the con- trary, the escaping vapor will be found to con- tain a smaller proportion of water than the orig- inal liquor Ijefore the distillation. The reason is mainly in the fact that water and alcohol are not equally volatile in the mixture; and, of course, the more volatile constituent — viz. alco- hol — distills over more rapidly than water, the less volatile constituent. The process of dis- tillation is very often actually employed by chemists when it is reqiiired to asceitain whether a given liquid rei)resents a single cojupoimd or a mixture. During crystallization, too, a single substance must obviously remain iinchange<l. Let, for example, a given amoimt of magnesium sulphate (Epsom salt) be dissolved in water and allowed to crystallize; any portion of the crystalline matter separating out must obviously have the same composition as the given salt. On the contrary, if every crystal of a giveft sub- stance contains two or more difi'erent com- pounds — say, magnesium sulphate and zinc sulphate — then any jjortion crystallizing out from the solution will be found to have a compo- sition quite different from that of the given sub- stance. The reason is mostly in the fact that the constituent substances of a mixture are not equally soluble in water; and, of course, the less soluble constituent will tend to ciystallizc out more rapidly than the constituent whose solubility is greater. It must, however, be remembered that certain mixtures, too, retain their chemical composition unchanged when divided into two or more frac- tions by distillation or crystallization. Thus, alcohol containing about 2 per cent, of water will remain absolutely unchanged if divided into fractions by distillation ; each fraction will still contain the same percentage of water. A cer- tain niixtire of water and nitric acid, and mix- tures of certain other substances, are known to behave in the same way. At one time these mix- tures were actually taken to be chemical com- pounds. This view, however, was discarded as soon as it was shown that the composition of such mixtures can be readily changed by chang- ing tlu' conditions under which the distillation or crystallization takes jilace. If such a mi.x- ture is, namely, distilled in an apparatus con- nected with an air-pump, the temperature at which the distillation takes place can be lowered by lowering the jjressure, and then the composi- tion will be found to have changed considerably if the liquid is examined after a part of it has passed over. No such thing can be observed iu the case of a single chemical compound. Wa- ter, for instance, may, by varying the pressure with the aid of an air-pump, be distilled at any ordinary temperature, yet its composition will of course remain unchanged. See Distillatiok. In accordance with the above considerations, a chemical compound may be defined as a homo- geneous substance u>hich can, hy suitable meth- ods, be broken up into elements, but ivhose com- position is not changed by fractional distillation or crystallizntion carried out under i^ariable con- ditions of temperature. This definition is suffi- cient for most purposes of theoretical and prac- tical chemistrj'. TnE Atomic Theoky. Once we have made cer- tain that we will not, by insufficient definition of our concepts, confound mixtures of substances with isolated chemical compoimds, we are ready to underiake the investigation of compounds, their physical and physiological properties, their composition, and their constitution and reactions — i.e. their chemical properties. The physical jiropertics, such as color, crystalline form, solu- bility in various solvents, the boiling or melting point, etc., serve the purpose of readily identify- ing known compounds. A knowledge of the physiological properties of compounds is desir- able, because compounds are often capable of therapeutic action, and may, therefore, be used in medicine. The theoretical chemist, however, is interested in all siich properties only inas- much as they are manifestations of the intimate nature of the compoimds characterized by them, and his principal aim is to find a precise expres- sion for the dependence of properties on chem- ical c<miposition and constitution. The composition of a compound is revealed by chemical .analysis, which shows (1) what the constituent elements are (qualitative analysis) and (2) in what relative quantities those ele- ments are contained in the compound (quantita- tive analysis). A remarkable law that governs the quantitative composition of compounds be- came known about the beginning of the Nine- teenth Century. This law, called 'the law of multiple proportions,' may be enunciated in the following form : There is for every chemical ele- ment a characteristic nimiber that represents its combining weight; and the composition of any chemical compound may he represented either by the combining weights of it.s elements or by simple multiples of those weights. Thus, using the combining weights as we know them at pres- ent, we may stjite the composition of a few compounds as follows: Carbonic oxide is com- posed of 12 parts of carbon and 10 parts of oxygen; carbonic acid of 12 parts of carbon and