Page:The American Cyclopædia (1879) Volume VI.djvu/148

 DISSOCIATION DISTILLATION taneous Decomposition of Bodies under the Influence of Heat." He snys : " By selecting a proper compound and heating it sufficiently, the distance between the molecules can be in- creased to such an extent that they will sepa- rate into their elementary condition. This is a spontaneous decomposition, not determined by any chemical action. I propose to call it the dissociation of compound bodies." In 1846 Grove showed that fused platinum could deter- mine the decomposition of water into its ele- ments. Deville repeated this experiment on a large scale by pouring fused platinum into water. He obtained an explosive mixture of hydrogen and oxygen, and believes that at the temperature of the fusion of platinum water is dissociated into its constituents. Analo- gous experiments can be performed on solids. Debray showed that when Iceland spar is heated in a tube from which the air has been exhausted, no decomposition takes place in mercury vapor at 350 C., and a scarcely per- ceptible decomposition in sulphur vapor at 440 C. ; but at 860 C. in vapor of cadmium it becomes very perceptible, and goes on till the tension of the liberated carbon dioxide becomes equivalent to 85 millimetres of mer- cury ; on raising the temperature to 1042 C. in vapor of zinc, more carbon dioxide is evolved. If, on the other hand, the apparatus be allow- ed to cool, the carbon dioxide is gradually reabsorbed by the quicklime, and a vacuum is reestablished in the apparatus. Lamy has applied these results to the construction of a pyrometer for the measurement of high tem- peratures. The apparatus consists of a porce- lain tube glazed on both sides, filled with pure carbonate of lime; one end of the tube is closed, the other connected with a manometer. By reading the volume of gas in the pressure gauge, and consulting the tables of tension, the temperature is determined. According to Deville, there is a tension of dissociation anal- ogous to the tension of vapors, and the evapo- ration of a liquid or the decomposition of a carbonate is subject to the same laws. In Fownes's "Chemistry," the exceptions to the law of molecular vapor occupying twice the volume of hydrogen are explained on the prin- ciple of the dissociation of the vapors at the high temperature required for the determina- tion of their vapor density. Several writers <>n geology, among them Fournet and T. Sterry Hunt, have had recourse to Deville's theory of dissociation to explain the origin of rocks and the action of forces in primeval chemistry. The force of chemical affinity appears to be suspended by great heat, so that at a high tem- perature, like that of the sun, we may imagine that chemical elements, such as oxygen, hydro- gi-n. chlorine, and sodium, can exist in the gaseous state, intimately mixed, but chemically uiieoiul.iiie.l. Many of the phenomena attrib- uted by Beraeihu to catalytic action are now explained on the principle of dissociation Since attention was called to the subject by Deville, a large number of bodies have been investigated with reference to the tension of dissociation, and the doctrine has been pushed to the determination of the temperature of combustion, also to the better understanding of efflorescence and the phenomena of va- porization. The dissociation of carbonic acid was accomplished by Deville by heat ; it has since been performed by Thenard by the elec- tric current. Carbonic oxide, sulphurous acid, hydrochloric acid, ammonia, and hydriodic acid have been dissociated by various chemists. It is the opinion of Dumas that Deville's theory of the tension of dissociation is as important to chemistry as Dalton's law of the tension of vapors was to physics. DISTILLATION (Lat. destillare, to drop), the conversion of a liquid or a solid into a vapor and condensing it ; usually applied to liquids. If sea water is boiled, the vapor which passes off leaves behind the salts and other substances held in solution, and by condensation in the atmosphere, or against cool surfaces, is con- verted into drops of pure distilled water. By artificial processes of a similar nature a volatile liquid may be separated from one less so, as alcohol, acetic acid, or ether from water with which they may be mingled. The volatile principles of plants may be extracted by water or other liquids, and by distillation separa- ted in a pure state, or dissolved in the liquic" used for extraction. Solid vegetable and ani- mal substances, by exposure to heat in vess more or less closed, undergo a process in whicl the organic compounds are destroyed, and their constituents recombined, partly in the form of volatile products, which may be collected as distillates, and partly as non- volatile residuum. By subjecting wood to such a process, char- coal, tar, pyroligneous acid, and naphtha are produced ; and by heating bituminous coal in close crucibles, illuminating gas, coke, coal, naphtha, and other products are separated. When solid substances, such as sulphur or cam- phor, are volatilized and condensed, the pro- cess, strictly speaking, is not distillation, because there is no collecting in drops ; but it is called sublimation. In the chemical laboratory dis- tillation is commonly conducted in glass retorts and receivers, the boiling taking place in the FIG. 1. Simple Distilling Apparatus. retort and the condensation in the receiver, which is usually cooled by the application of water or ice to the exterior. The simplest apparatus of this kind is shown in fig. 1.