Pipettes
These are glass instruments used for measuring liquids in drops, and so constructed that the flow of the liquid from them is under the complete control of the operator. They may be made in any form which may be suggested to adapt them to special purposes; but pipettes for general use are usually constructed as follows: Fig. 1 is an ordinary pipette, and consists of a small cylinder of glass with an upper and lower tube, the lower end terminating in a fine orifice for the discharge of the fluid, and the upper end adapted for the finger er thumb, by which the outward flow can be instantly arrested.
This is filled by the suction of the mouth. Fig. 2 is made on the same principle, having a fine orifice (b), and a thumb-hole (a), but fitted with a mouth and stopper on the upper side, for convenience of filling, or insert- Fig. 1. ing a measured quantity of liquid. The lower side being flat, to allow of the instrument being laid down without risk of waste of contents. a Fig. 2. b A pipette affords also a ready means of separating two liquids, too small in quantity to allow of separation by decantation or other methods usually employed. To this end, the upper or lower stratum of the mixed liquids (oil and water, for instance), may be drawn by the mouth into the pipette; or the whole may be sucked into the pipette, and the lower stratum allowed to flow out.
Graduated pipettes of various forms, especially useful in acidimetry, &c., will be found described in No. 82. These instruments are also useful, and in many cases indispensable, in conducting delicate tests. flannel to a frame, which can be laid over the mouth of a pan; or by laving the flannel across the mouth of a coarse hair-sieve. Concentrated infusions and decoctions, being usually weak tinctures, may be filtered as tinctures. (See No. 17.) Viscid vegetable solutions may be clarified (see No. 1357); or may be made to filter rapidly by the addition of acetic, sulphuric, or other strong acid.