Licence: Public Domain Mark
Credit: Qualitative chemical analysis / by C. Remigius Fresenius. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![into gi'oups. Some of tlio precipitated sulphides exhibit a characteristic colour indicative of the individual metals which they respectively contain. The great facility with which sulphuretted hydrogen is de- composed renders this substance also a useful reducing agent for many compounds; thus it serves, for instance, to reduce salts of sesquioxide of iron to salts of protoxide, chromic acid to the state of sesquioxide of chromium, &c. In these processes of reduction, the sulphur becomes sepai-ated in the form of a fine white powder. Special circumstances determine as to whether it is better to employ the sulphuretted hydrogen in the gaseous form or in aqueous solution. III. Bases and Metals. § 33. Bases are divided into oxygen bases and sulphur bases. The former result from the combination of metals or of compound radicals analogous to them with oxygen, the latter from the combination of the same substances with sulphu.r. The oxygen bases maybe classified as alkalies, alkaline earths, earths proper, and oxides of the heavy metals. The alkalies are readily soluble in water; the alkaline earths dissolve with greater diflB.culty in it; and magnesia, the last member of the class, ib only very spar- ingly soluble. The earths proper and the oxides of the heavy metals are insoluble in water or nearly so (except protoxide of thallium). The solutions of the alkalies and alkaline earths are caustic when sufficiently concentrated; they have an alkaline taste, change the yellow colour of turmeric paper to brown, and restore the blue tint of reddened litmus paper; they saturate acids completely, so that even the salts which they form with strong acids do not change vegetable colours, whilst those with weak acids generally have an alkaline reac- tion. The earths proper and the oxides of the heavy metals likewise combine with acids to form salts, but, as a rule, they do not entii-ely take away the acid reaction of the latter. The sulphur bases formed by the combination of the metals of the alkalies and alkaline earths with sulphur are soluble in water. The solutions have a strongly alkaline reaction. The other sulphur bases are insoluble in water. With sulphur acids, all sulphur bases form sulphur salts. a. Oxygen Bases. a. Alkalies. § 34. 1. Hydrate of Potassa, KO,HO [KHO], and Hydrate of Soda, NaO,HO [NaHO]. The preparation of perfectly pure potassa or soda is a difficult opera- tion. It is advisable therefore to prepare, besides perfectly pure caustic alkali, also some which is not quite pure, and some which being free from certain impurities may in many cases be safely substituted for the pure substance.](https://iiif.wellcomecollection.org/image/b21966953_0074.jp2/full/800%2C/0/default.jpg)
