The best method of weighing out exact quantities may be here described for those to whom it is not already known, as much time may be wasted by attempting it unsystematically. The material is of course weighed in a basin, which together with the weight which is desired of the material, is exactly counterpoised by weights in the other pan. Where many weighings of the sort have to be made, it saves time to keep one particular basin for the purpose, which should be properly marked[154]; and to make a counterpoise of lead or brass exactly equal to it in weight, so that it is only necessary to add weights corresponding to the quantity required to be weighed out. Supposing now, that it is a liquid extract which is to be weighed, a sufficient quantity is introduced into the basin with a pipette, to slightly exceed the required weight. The pipette is now emptied, and a small quantity is withdrawn with it from the basin. If the basin is still too heavy, the pipette is emptied, and the process repeated until the basin is too light. The true weight now lies between that in the basin and the small quantity retained in the pipette, from which extract is added till the basin is again over-weighed, and the same process is repeated, each time reducing the margin, till a sufficient approximation is obtained. It is not necessary in weighing out the sample, to be accurate to a single milligram; but with practice, this amount of accuracy is easily attained. If the material is solid, a spatula is substituted for the pipette. The weighing of liquid or pasty extracts should be as rapid as possible, as they lose weight on the balance by evaporation. [154] Porcelain basins may be indelibly marked by writing on them with an ordinary iron ink, and heating strongly with a blowpipe. Liquid extracts are most easily dissolved by placing a large funnel in the neck of a liter flask, and after pouring a little boiling water into the flask, holding the basin inclined in the funnel, and washing out its contents with boiling distilled water from a glass wash-bottle, or a perfectly clean copper kettle, till the flask is filled to the mark. The flask is now covered with a small beaker, which must hang loose on its neck, without resting on its shoulders, and is rapidly cooled by placing it under a cold water tap, to a temperature as little above 15° C. as possible, and is then filled up to the mark on the neck with cold water, and well mixed by shaking very thoroughly. Solid or pasty extracts are dissolved in a beaker by stirring with successive quantities of boiling water, which are poured off into the flask, leaving the undissolved matter in the beaker. When the flask is nearly full, if any small portions remain undissolved or insoluble, they may be rinsed into it with the last portions of hot water, and the flask is now cooled and mixed as already described. [Illustration: FIG. 65.--Procter’s Extractor.]