ANALYSIS. After a compound has been iso lated and purified by repeated distillation or crystallization, its physical properties (especial ly the boiling or melting point) are carefully determined, and then it is analyzed. The analy sis of carbon compounds usually involves the determination of carbon and hydrogen, and often of nitrogen. Carbon and hydrogen are deter mined simultaneously by heating a known amount of the compound in a glass tube with copper oxide, which oxidizes all the hydrogen into water vapor and all the carbon into carbonic acid. The process is for evident reasons termed a com bustion. The products of the combustion are col lected, respectively. in sulphuric acid and in caustic potash, and their weights are carefully de termined, the percentages of hydrogen and of car bon in the given compound being calculated from those weights. A second combustion is required to determine the amount of nitrogen that may he present in the compound. The combustion yields the nitrogen in the tree state, the gas being col lected in a graduated tube •hieh shows the vol ume prodeced, and from this the percentage, by weight. of nitrogen in the given compound is found by a simple calculation. A direct deter mination of oxygen is unnecessary: after the per centage: of carbon. hydrogen, and nitrogen have been determined, that of oxygen becomes evident. In the case of organic compounds of silver the amount of the latter nwy often be determined by simply heating a known quantity of the com pound in a crucible: the metal then remains be hind assuvh, a Ild is weighed directly, while the rest of the compound burns away. The amount of
sodium or potassium in an organic compound may be determined by heating a known quantity of the latter with strong sulphuric acid, pure sodium or potassium sulphate being thus pro duced, and from the weight of this the percentage of .metal in the compound is readily calculated. The halogens (chlorine, bromine, and iodine) are best determined in Organic compounds by heating these in scaled glass tubes with fuming nitric acid and silver nitrate, the product being a halo gen compound of silver, from the weight of which the percentage of hydrogen in the given organic compound is found by a simple calculation. The determination of sulphur, phosphorus, and other elements that may sometimes be present in car bon compounds need not be described here, the purpose of the present sketch being merely to convoy' a general idea of the ways in which it is possible to ascertain the composition of organic substances, and not to give detailed specific in forMation: such information should be sought ill special works on chemical analysis.