The identification of starches by means of their microscopical appearances is tolerably simple, as each particular kind possesses characteristic differences. _Gelatine._—Samples of gelatine should be examined for water and ash. The water should not exceed 16 per cent. and the ash 2·7 per cent. The relative strengths of the jellies formed with water should also be compared. This may be done in the way indicated above for starches. Raw materials intended for the preparation of gelatine may be similarly examined. They yield the whole of their gelatine on heating with water. The effect of prolonged heating on solution of gelatine should be borne in mind. The amount of residue left after treatment with water should be carefully determined. _Soaps._—The soaps most suitable for use in paper-making {209} are those known as curd and mottled, the former being chiefly used for the fine qualities. They should be carefully examined for free alkali and unsaponified fat. Full descriptions are contained in Lant Carpenter’s ‘Treatise on Soap-making.’ _Dyes, Pigments, Loading Materials, &c._—These substances are best examined from a physical and tinctorial point of view, by comparison with specimens of undoubted purity and excellence. The chemical examination is both difficult and somewhat misleading. A microscopical examination will often throw much light on their composition. For comparing the tinctorial power of different samples, weighed quantities should be ground up with starch or some other white powder. In this way some idea of the intensity of colour can be readily arrived at. {210} CHAPTER XV. SITE FOR PAPER-MILL, WATER-SUPPLY, WATER PURIFICATION, ETC. In choosing a spot on which to build a paper-mill, the manufacturer has to take into consideration several very important circumstances. Chief of these is the necessity for having a large supply of water at command. Not only is a large quantity needful, but it should be free from impurities, such as suspended matter and iron. The former, it is true, can be removed by settling and filtration; the latter cannot, and is liable to injuriously affect the colour of the paper. Again, as a question of economy in working, it is advantageous to have convenient water-power; therefore for this, as well as for the former reason, paper-mills are usually situated on the bank of a stream. In choosing such a site, paper-makers are probably also influenced by the fact that it affords a ready means for the removal of impurities. In properly conducted mills, where suitable apparatus is employed for evaporating the liquors in which the raw material has been boiled, the stream should not be polluted to any very great extent. Generally speaking, the greater the pollution, the more are valuable materials being lost to the manufacturer. It is obvious that the site for a mill should also be chosen with reference to its proximity to means of transit for the raw and manufactured materials.