This latter contains an aperture running in the direction of its axis, and the whole is arranged so as to form a tight joint.

Fig. 90.

Distilling Part 3 400102

When the substance distilled attacks cork and rubber, the neck of the retort is drawn out to a sufficient length to allow the tube that terminates it to enter the refrigerator to some depth. The rubber with which the 2 parts of the apparatus are connected is thus nearly out of the range of the vapours.

It is very evident that the still may he replaced, and advantageously too, in many cases, by any other spherical vessel with a narrow neck. In this case the receiver is closed (Fig. 91) by a cork or rubber stopper containing an aperture that is traversed, through slight friction, by a glass tube. This latter is so bent that the angle formed by its 2 branches shall correspond to the inclination that is given to the refrigerator. The external extremity of the tube is connected with the refrigerator by means of one of the arrangements described above for the neck of the retort. As for the internal extremity, it is well, especially if the tube is narrow, to bevel it off so as to facilitate the flow, drop by drop, of the condensed liquid which accumulates therein, and which, without such a precaution, might be carried along by the vapour toward the refrigerator. Moreover, in the case of a liquid that would attack the joints, the bent tube that fits into the neck of the receiver may be that of the refrigerator itself.

Fig. 91.

Distilling Part 3 400103

One of the best forms of still for the photographer to employ consists (Fig. 92) of a tin can or bottle in which the water is boiled, and to this a tin tube is adapted by means of a cork, one end of this tin tube terminating in a coil passing through a tub or other vessel of cold water. A gas burner, as shown, is a convenient source of heat, and in order to ensure a complete condensation of the vapour, the water in the cooling tub must be changed now and again.

Sometimes the vapour is condensed by being allowed to play against the inside of a conical cover which is adapted to a saucepan, and is kept cool by the external application of cold water; and in this case the still takes the form represented by Figs. 93, 94, and 95; such compact and portable stills being largely employed in Ireland for the private manufacture of whisky.

Fig. 92.

Distilling Part 3 400104

Fig. 94.

Distilling Part 3 400105

It is scarcely necessary to say that the condensed water trickles down on the inside of the cone, and flows out at the spout.

An extemporised arrangement of a similar character may be made bypassing a tobacco pipe through the side of a tin saucepan as shown in Fig. 93, and inverting the lid of the saucepan; if the lid is now kept cool by frequent changes of water inside it, and the pipe is properly adjusted so as to catch the drippings from the convex side of the lid, a considerable quantity of distilled water may be collected in an hour or so.

The proportion of solid impurities present in water as ordinarily met with is extremely variable; rain water which has been collected toward the end of a storm contains only a minute fraction of a grain per gallon, while river or spring water may contain from less than 30 gr. per gallon or so upward. Ordinary sea water generally contains 3-4 per cent. of saline matter, but that of the Dead Sea contains nearly 1/4 of its weight of salts.

Fig. 93.

Distilling Part 3 400106

Fig. 95.

Distilling Part 3 400107

The 3 impurities of water which most interest the photographer are lime or magnesia salts, which give the so-called hardness; chlorides (as for example, sodium chloride or common salt), which throw down silver salts; and organic matter, which may overturn the balance of photographic operations by causing premature reduction of the sensitive silver compounds. To test for them is easy. Hardness is easily recognisable by washing one's hands in the water, the soap being curdled; but in many cases one must rather seek for a hard water than avoid it, as the tendency of gelatine plates to frill is far less in hard water than in soft water. It is, indeed, a common and useful practice to harden the water used for washing by adding 1/2-1 oz. of Epsom salts (magnesia sulphate) to each bucket of water. Chlorides - sodium chloride or common salt being that usually met with - may be detected by adding a drop or two of silver nitrate to half a wineglassful of the water, a few drops of nitric acid being then added. A slight cloudiness indicates a trace of chlorides, and a decided milkiness shows the presence of a larger quantity.

If it is wished to get a somewhat more definite idea of the amount, it is easy to make up a series of standards for comparison, by dissolving known weights of common salt in distilled or rain water, and testing samples of them side by side with the water to be examined.

Organic matters may be detected by adding a little silver nitrate to the water, filtering off from any precipitate of silver chloride, and exposing the clear liquid to sunlight; a clean stoppered bottle being the most convenient vessel to use. The extent to which a blackening takes place may be regarded as approximately proportionate to the amount of organic matter present.

Filtration on a small scale is not altogether a satisfactory mode of purifying water, as organic impurities often accumulate in the filter, and enter into active putrefaction when hot weather sets in. (Photo, News.)

The simple apparatus shown in Fig. 96 works admirably, and is very convenient, a is a common tin saucepan, with a small hole in the side, for a tobacco-pipe; 6, a "steamer," on top, with a bottom like an inverted cone, 1 in. of wire being soldered at the apex. A gas jet (Bunsen's, if possible) boils the water in the saucepan; the ascending steam is condensed on the lower surface of the steamer, runs down to the point of the wire, down the pipe into the bottle. A small jet of cold water keeps b cool.