Chemical Manipulations. Some of the operations employed in the preparation and use of chemicals have already been given at the commencement of this book (see No. 1 (Manipulations)); but, as the work progressed, it was deemed advisable, for the sake of greater precision, to add further directions for special manipulations, and descriptions of indispensable apparatus.

3831. Separating Funnels

3831. Separating Funnels. These are glass funnels furnished with a stop-cock, and are used for separating mixed fluids of different densities. The mixed liquid is poured in-

Fig. 1.                              Fig. 2.

to the funnel, and, after sufficient time has been allowed for the heavier liquid to settle, it can be drawn off by opening the stop-cock, closing it immediately after the heavy liquid has passed. Fig. 1 represents a separating funnel, such as is used for ordinary purposes; but for separating a mixed liquid containing ether or other volatile fluid, a funnel, closed with a stopper similar in construction to Fig. 2, is employed to prevent evaporation while the heavier liquid is settling. For very small quantities a pipette (see No. 3832 (Pipettes)) is the best instrument.

3832. Pipettes

3832. 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 or 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 (6), 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 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, etc., will bo found described in No. 82. These instruments are also useful, and in many cases indispensable, in conducting delicate tests.