The filtrate obtained on precipitating out the three metals just spoken of is treated with sul phuretted hydrogen. This precipitates a second group of metals, which are separated from one another by methods analogous to those employed for the first group. The filtrate obtained on pre cipitating out the metals of the second group is usnally treated with ammonium sulphide, and the filtrate from the ammonitun sulphide group with ammonium carbonate. Thus the metals that may be present in the original substance are separated into several groups, and then special .methods are employed to separate and test for the several metals composing each group.
The acid radicals are tested for in a somewhat similar manner, but usually less systematically; because by the time all the metals present have been identified, the analyst usually is able to ex clude the possibility of the presence of a large number of acids.
The spectroscope (q.v.) is usually applied to identify the metals potassium and lithium, and is quite indispensable when substances are to be examined to ascertain whether they are in the purest condition possible, since the instrument is capable of revealing the presence of the merest traces of substances.. See SPECTRUM ANALYSIS.
The system of analysis usually followed may be carried out mechanically and almost without intelligence, if the substance examined contains only the more familiar metals and acids, and those in considerable quantities. In fact, quali tative analysis is criticised by teachers on this account, when used as a discipline, or as a means of acquiring a scientific knowledge of chemistry.
The ordinary scheme, however. overlooks even some elements of common occurrence, as tita nium; and when the chemist has to take into consideration small amounts and the less famil iar elements, all his chemical knowledge and acuteness find full field for exercise.
It may be seen from the above that the chem ist relies on two sets of properties for the identi fication of a substance. First, those that belong to the substance itself under ordinary condi tions; for example, the yellow color and the lightness of sulphur. Such properties may be called properties of condition. On the other hand. if sulphur is heated sufficiently without access of air, it assumes the form of a red vapor; if heated with access of air, it forms with the oxygen of the air a colorless gas possessing a characteristic odor. The first of these changes is physical; the second, chemical. Physical or chemical changes may thus serve to bring out certain properties that are just as characteristic of the substance as the properties of condition. Such properties may be called properties of re action. They are far more numerous than prop erties of condition, and far more useful to the analyst. In the case of sulphur, for instance, the properties of condition are only apparent when the sulphur is in a nearly pure form: hut the two properties of reaction just mentioned as an example enable us to identify sulphur even when mixed with so much foreign matter that the characteristic color and lightness arc quite masked.