A chemical balance suitable for the preparation of standard solutions and general analytical use, is shown in Fig. 16. The beam is provided with steel or rock-crystal knife-edges at the centre, which are supported on agate planes, and similar edges _a_ support the pans. Except at the moment of weighing, the beam, and in good balances the pans also (at _b_), are steadied by supports raised by turning the milled head _c_. The long pointer _d_ moving over a scale, shows when the beam is horizontal; but the weighing is performed, not by waiting till the balance comes to rest, but by noting when the oscillations are equal on each side of the zero point. The _weights_, which should run from 50 _grm._ downwards, are usually of brass (preferably gilded) down to 1 _grm._, while the fractions to 0·01 _grm._ are of platinum foil. Milligrammes and fractions are weighed by a "rider" of wire weighing 0·01 _grm._, and moved along the beam (which is graduated for the purpose like a steelyard) by the arms _e_. A fair balance should turn distinctly with 0·001 _grm._, and a good one with 0·0001 _grm._ If equal weights are placed on each pan, they should of course balance, and if changed side for side the balance should be maintained. If not, the arms of the beam are unequal. Weights always have trifling errors, but if by a really good maker, these are generally so small that they may be disregarded except in very delicate researches. The weights should always be placed on the scale in regular order, beginning with the heaviest, and it is well to accustom oneself to reading the weight by the vacant places in the box as well as by the weights on the scale. [Illustration: Fig. 16.] While of course it is most important, and for accurate work essential, to have as good a balance as possible, much may be done in technical work, even with a good pair of druggists' scales; and most standard solutions may be bought ready made; while from two or three accurately adjusted solutions many others may be made volumetrically.