Stock solutions may also be prepared for unstable substances, but sufficient only for one week should be made, up, bearing in mind the fact that concentrated solutions keep much better than dilute solution (this is the case with sulphite of soda and sodium sulphide).
To avoid the necessity of handling a large bottle when only a small quantity of liquid is required, the stock solutions may be stored in bottles fitted with either a glass tube outlet at the bottom or a piece of glass tubing, which, after passing the neck of the bottle, is bent round to form a syphon (see Figs. 162 and 163). In either case, the outlet is connected to a glass jet by means of a length of rubber tubing, which is closed by a spring clip or by the insertion of a glass bead. In the latter method, the liquid is delivered by pinching the rubber tubing round the bead. 1 When baths are prepared immediately before use by the admixture of stock solutions, insuffi cient care is often taken to secure a perfectly homogeneous mixture. This cannot be obtained merely by pouring out the solutions (which have been measured together in a graduated measure) into another receptacle, for the mutual penetra tion of many solutions is much slower than is often realized. The mixing should be carried out in a measure which is closed securely with the palm of the hand, and then inverted several times, or the liquid decanted several times from one container to another.
279. Commercial Preparations. A large num ber of preparations in various forms have been put on the market to relieve amateurs of the necessity of weighing the various chemicals for the solutions- (a) Solutions ready for use, or concentrated solutions to be diluted with water as directed.
(17) Dry powders contained in boxes, sealed or stoppered bottles and tubes, and wax-paper packets.
(c) Compressed tablets, to be crushed between clean paper before introduction into the volume of water indicated.
(d) Homogeneous pastes, prepared by mix ture with glycerine or dextrine solutions, de livered from flexible metal tubes similar to those employed for tooth pastes. A definite length of the paste corresponds with a certain volume of the particular bath (Guillemot, 'goo).
(e) Paper ruled off into squares and impreg nated on both sides, each square corresponding with a certain volume of the bath (Zi6gler, Igor).
Owing to the diversity of the formulae used in these preparations, the only advice which can be given is to adhere strictly in each case to the accompanying instructions.