Fig. 13.
Fig. 14.
Pipettes ([Fig. 12]) are tubes with a mark on the stem by which exact quantities of liquid can be taken. Several holding 5, 10, 20, and 25 c.c. are necessary, and one holding 10 c.c. and divided into tenths is advisable. Most important of all is the burette ([Fig. 13]). If only one is to be had, it must be a Mohr's burette with a glass tap, but as alkaline solutions are apt to set glass taps fast, it is well to have one with a tap, and another with a pinchcock ([Fig. 14]). They should hold 50 or 25 c.c. and be divided into tenths. The burette in use is fixed in a stand ([Fig. 15]) and filled up to the top of the graduation, and the quantity of solution delivered is then shown by the scale. It is usual to read by the under side of the hollow of the liquid, keeping the eye carefully level with it.
Fig. 15.
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.