Dish, A. Feb. 3 9.4210 grams. 5 9.4225 6 9.4230 7 9.4200 Platinum vessels and apparatus lose, and porcelain ones slightly gain, weight with continued use. The special details of the work is given under each assay; certain general instructions will be given here. ~Solution.~--It is not always necessary to get the whole of the mineral in solution, provided the body sought for is either completely dissolved or altogether left in the residue. It is often only by a qualitative examination of the solution (or residue, as the case may be) that the assayer can satisfy himself that it is free from the substance sought. But previous experience with the same kind of ore will show to what extent this testing is necessary. Solution is generally best effected in flasks; but where the resulting liquid has afterwards to be evaporated to dryness and ignited, evaporating dishes (fig. 12) are used. With them clock glasses are used as covers during solution to avoid loss through effervescence. Evaporating dishes are also best when an insoluble residue has to be collected, since it is difficult to wash out most residues from a flask. Bumping occurs less frequently in dishes than in flasks. [Illustration: FIG. 12.] After the addition of the acid, and mixing by agitation, the vessel containing the substance is heated. This is best done on the "hot plate" (fig. 13). This consists of a slab of cast iron about half or three-quarters of an inch thick, supported on loose fire bricks, and heated by two or three ring burners (figs. 14 and 15). The burners are connected to the gas supply by means of _lead_ tubing, to which they are soldered. Flasks and dishes after being put on the plate are not further handled until solution is complete or the evaporation is carried to dryness. The hot plate is contained in a cupboard so as to be out of the reach of cold draughts. [Illustration: FIG. 13.] [Illustration: FIG. 14.] [Illustration: FIG. 15.] The action of the acids and other solvents is described in the chapter on Reagents. ~Precipitation.~--In precipitating add sufficient of the reagent to complete the reaction. The student must be on his guard against adding a very large excess, which is the commoner error. In some reactions the finishing point is obvious enough; either no more precipitate is formed, or a precipitate is completely dissolved, or some well-marked colour or odour is developed or removed. In those cases in which there is no such indication, theoretical considerations should keep the use of reagents within reasonable limits. The solutions of the reagents (_see_ REAGENTS) are generally of five or ten per cent. strength. A small excess over that demanded by theory should be sufficient. [Illustration: FIG. 16.] [Illustration: FIG. 17.] [Illustration: FIG. 18.] [Illustration: FIG. 19.]