started and stopped: it is advisable occasionally to remove from the interior of the still, and the supply-tube connecting it with the condensing vessel, the deposit which accumulates by evaporation of the water.

The accompanying sketch (fig. 42) renders a lengthened description of the apparatus unnecessary: it represents the apparatus in section. A copper-still (A) is heated by the flame of a large-sized Bunsen-burner or by a ring gas-burner, the steam passing off from the top of the still through a tin tube bent into a spiral form in the condensing-tub (B) which is

made of galvanised iron; from this condenser the distilled water flows into a large stoneware vessel (C), with a tap below from which the water is drawn when required. A long syphontube (D) serves as a gauge. The condensing-tub (B) has three tubes let into the side facing the still; one of these (d), placed about an inch above the bottom of the vessel, is the

inlet for cold water from a cistern whose water level must be higher than that maintained in A and B; this level is marked by the single dotted line, and is kept constant by the overflowtube e, which is connected with a pipe leading to a sink or drain. Just below this overflow-tube is a short tube (ƒ), which is on a level with another (g), opening into the side of the still; by connecting these two tubes with an indiarubber joint, a supply of warm water from the upper part of the condenser is furnished to the still, keeping the level of water in it constant: a small glass jet drawn out from a piece of glass tubing is fitted into this connecting-tube, and has been found sufficient to prevent a too free circulation of water between the still and condenser. This still is supported on an iron ring, projecting as a bracket from the condenser. By having two taps in the inlet-tube (d), or by using two clamps on the india-rubber joint which connects it with the supply-pipe from the cistern, much time is economised, as the flow of water may be regulated once for all by one tap or clamp, the other being used only for stopping and starting the stream. The copper-still should be furnished. with a lid screwing upon its mouth, for convenience in removing the deposit which collects within, and into this lid should be fastened a brass tube carrying a screw-union by which the tin condensing-pipe may be fastened steam-tight to the still. The wooden stand for the still and storing jar is conveniently fitted with shelves and doors to serve as a cupboard.

For tests of purity see Remark 41 (526).

Such a copper-still, 23 inches in circumference below and 8 inches in height, when filled to a height of 4 inches and heated by the largest-sized Bunsen-burner, yields 2 litres of distilled water per hour.

RECOVERY OF THE METALS FROM SILVER AND PLATINUM RESIDUES.

When AgNO, or PtCl has been added to a solution the liquid is not to be thrown into the sink. 507. Silver Residues.-If AgNO3 has been added to a liquid it must be emptied into a vessel labelled "Silver Residues," containing strong HCl which precipitates AgCl. When the vessel is full the AgCl is allowed to settle, and the liquid decanted from the AgCl; more HCl is then poured in, and the vessel is again ready for use. When a sufficiently large quantity of AgCl has been formed, let it settle, decant off the liquid, and wash the precipitate well by decantation; pour it upon a filter, and dry upon the filter. One of the two following methods may then be employed to obtain metallic Ag from the AgCl:

1. Mix the AgCl with twice its weight of a mixture in equal proportions by weight of Na,CO, and K,CO,; place this in a clay crucible of such a size that the mixture only half fills it, and heat in a furnace until the mass becomes liquid; maintain it in a fused condition for about five or ten minutes, remove the crucible, and tap its bottom several times gently upon a brick to cause the fused particles of Ag to unite. Allow the crucible to cool, break it up, and wash the button of Ag, which will be found at the bottom, until it is quite free from adhering salts.

This button is then dissolved by heating it in a flask with pure strong HNO, previously diluted with about half its measure of water; the solution is evaporated to dryness, the residue dissolved in a little water and once more evaporated to complete dryness. This AgNO, is then dissolved in the right proportion of water to furnish reagent No. 48 (par. 525).

2. A readier method for decomposing the AgCl consists in placing it in a dish with a little water acidified with dilute

* Most conveniently a large bottle, with a funnel in its neck.

H2SO4, and laying upon the AgCl some strips of Zn free from Pb; after some hours the Ag will be separated as a black spongy mass. This is well washed by decantation with hot dilute H2SO4, and then washed also by decantation with boiling distilled water till free from the H2SO4; the Ag is dissolved in HNO3 as directed in the first process, the solution evaporated to dryness, and the AgNO, used for preparing the reagent. Any residue insoluble in HNO, will be unreduced AgCl; this may be filtered off and added to the silver residues for subsequent reduction.

508. Platinum Residues.-Liquids to which PtCl has been added are poured into a vessel * labelled "Platinum Residues;" when a sufficient quantity has been obtained the liquid and precipitate are poured into a porcelain dish, evaporated to dryness, and heated strongly for some time; as soon as the dish is cool the residue is boiled with water, which is decanted, and upon the residue some solution of oxalic acid is poured; this is evaporated to dryness and the residue strongly ignited. The residue of metallic Pt is once more washed with boiling water, and is then dissolved by heating it with HCl to which one-third its measure of HNO3 has been added; the solution is evaporated to dryness, the last part of the process being performed over a water-bath. HCl is then poured in, and it is once more evaporated to dryness and heated for some time on the water-bath; this residue when dissolved in water forms the reagent No. 28 (525).

* Conveniently a large glass bottle, with a funnel in its neck.

DIRECTIONS FOR PREPARATION OF REAGENTS.

Note. The reagents required for chemical analysis are much more readily obtained at the present time than they were in former years. They can usually be purchased in a pure state, and therefore processes of preparation, which formerly of necessity occupied a large portion of Manuals of Practical Chemistry, are here omitted. It will be found usually more economical to purchase pure chemicals than to prepare them.

509. Many reagents are required in a dissolved or diluted state; it is a matter of much importance that these solutions should be made of a proper strength, and that the methods of preparing them should be as rapid and simple as possible.

The strength of the solutions is chosen somewhat arbitrarily; indeed, the strength of a solution may be varied with advantage according to the different purposes for which the reagent is to be employed. It is found, however, that practically a solution of one strength may usually be made to serve all analytical purposes. The proportions of liquid or solid to be mixed with or dissolved in a certain quantity of water, which are given hereafter, are those (or very nearly those) recommended in standard works on analysis. It is not intended, however, to assert that they are for all purposes the best, but merely that they are practically useful.

510. A good and economical store bottle for liquid reagents is the "Winchester Quart" bottle, a stock of which generally accumulates in a laboratory, being continually introduced filled with strong acids. The following methods of filling these have been found simple and easy of execution.

The process of preparing solutions divides itself naturally into the "dilution of liquids" (511), and the "solution of solids" (512).

511. DILUTION OF LIQUIDS.

In the following lists the proportions by measure are stated in which the liquid and distilled water are to be mixed; these preparations are roughly, but sufficiently accurately,