salt is added until the water becomes so heavy that the soap rises to the surface, whence it is removed into moulds or frames and allowed to cool, when it is cut into bars for sale.

Soft soap is made in the same way, using potash instead of soda, and, generally, a large quantity of trainoil. Castile soap is pure soda soap, and the bluish or red mottled appearance is produced by stirring in some sulphate of iron (green vitriol); when new, it is of a bluish colour, but gets red by exposure to the air.

Oils and fats combine with the oxides of several of the metals, and these compounds are all soaps, but by far the greater number of these are useless as such, being quite insoluble in water. A combination of oxide of lead with olive oil forms a firm solid substance, known as lead plaster, which, with the addition of a little resin, is used in surgery, and, when spread upon linen or calico, forms the common adhesive plaster.

Oils and fats all consist of a combination of organic acids with glycerine. When these fats are boiled with soda, potash, or metallic oxides, a combination of the oxide and fatty acid takes place, forming in the ordinary soap, stearate, oleate, and margarate of soda-these constitute soap. The glycerine is set free, and, when purified, forms a sweet, oily, colourless fluid, very similar to syrup, but not so sweet; it has lately been used for several purposes, especially as a remedy for "chapped hands:" a soap called "glycerine soap" has lately been used for the same purpose; it is made without separating the glycerine. Fancy soaps are made from good yellow soap, coloured and scented. White Windsor soap is a pure kind of soap made of tallow and soda; the brown Windsor soap is coloured to imitate old soap, for soap when recent is very soft, and contains much water, it is, therefore, found much more economical to keep it till it gets quite hard from the loss of water; this, however, produces a brownish colour. The history of "Old Brown Windsor Soap" is, therefore, similar to that of brown brandy-both being colourless when new, and

both being coloured to imitate age, and carried to such a ridiculous extent, that they become totally unlike the articles they are intended to imitate.

Before the process for making soda from sea-salt was in general use, barilla was the substance employed to make hard soap with. It was an impure kind of soda procured by burning sea-weed, and contained a certain proportion of iron, and in the ley which was made by adding lime there was a dark-coloured sediment; this on being stirred up with the soap produced a dark mottled appearance, and gave rise to the term Mottled Soap.

Since the introduction of soda, it also has to be imitated to keep up an article of a well-known quality.

Soap is quite soluble in spirit and also in warm distilled water, but in ordinary water it is to a great extent insoluble, producing a milky solution; this is owing to the lime contained in the water which forms an insoluble soap; soap has therefore been used as a test of the hardness of water. A solution of soap in spirit being dropped into a specimen of the water, the amount of sediment shews its hardness.—Boys' Book of Industrial Information.

LEATHER.

TANNING is the name given to the process for converting the skins of animals into leather, by combining them with a substance called “Tannin." This exists in many vegetable substances, such as oak-bark, gall-nuts, sumach, &c., &c.; all of these, and many more, are used for tanning, but, on account of its cheapness, oak-bark is the usual substance employed.

It is tannin which gives the quality of astringency to many vegetables, and this very taste of astringency is produced by a partial contact of the tannin with the surface of the mouth.

The skins (called "hides" or "pelts") are first freed

from all loose pieces of flesh, fat, or skin; the hair is then removed by soaking them in lime and water. The skins are next laid in the "tanpit," between layers of crushed oak-bark, until the pit is nearly full; water is then pumped in, and the whole is allowed to remain for several weeks or months (according to the thickness of the skin), during which time, however, the skins are changed in position, by removing them from one pit to another with fresh bark in it, so that those taken from the top of the first, are placed at the bottom of the next; and this is done from time to time, in order that all may receive the same pressure and strength of tanliquor. Very thick bodies take a year to tan perfectly in this way, and consequently many processes have been tried to quicken the operation; but the leather made most slowly seems to wear the best, and consequently fetches a higher price.

Skins which are thin, and to be used for fancy-work, as for book-binding and glove-making, are either tanned with "sumach," or with alum and salt made into a paste with flour and yolk of eggs; this is put into a tub, and the mixture and skins worked together till they are thoroughly united.

Besides boots and shoes, leather is used for the harness of horses, covers for seats, gloves, and innumerable other articles.

The

For some uses the leather is required to be very thin, and of exactly one thickness. This is obtained by the process of splitting, for which a machine is used whose exactness is such, that one slice is taken from the inner part of the skin without cutting a hole in any part. skin is stretched tightly round a roller, which slowly revolves against a straight knife-edge, fixed at a certain distance from it, according to the thickness of the skin, and which is passed by the machine backwards and forwards, cutting the skin a little further each time.— Boys' Book of Industrial Information.

PAPER.

THIS important article of civilization is made from rags, and other fibrous materials, according to the kind of paper required, the finest and thinnest white paper (such as bank-notes and foreign writing paper) being made of old clean linen rags; thicker paper, of linen and cotton rags; newspapers, of coarse linen, white rope, Esparto grass, palm, and the ground fibre of the aspen and poplar, while brown paper is made of all sorts of ropeyarns, or sacking, and some kinds have a considerable amount of straw bleached and worked up in them, largely mixed with gypsum, clay, and ochre, to give weight and colour. The rags are first sorted and cut up into small pieces; they are then placed on a revolving screen to separate all dust; after which they are put into cylindrical or spherical boilers, where, mixed with lime and soda, and constantly agitated, they are boiled by steam at high pressure for many hours, until all grease and dirt are entirely dissolved. They are next put into the washing-machine (through which a stream of water runs) where they are torn by a heavy roller, having iron knives fastened to its edge. or surface, which work, as the machinery is turned, against knives of a similar description fastened to the bottom of the cistern. When the rags are thoroughly washed, and at the same time torn to a coarse pulp, they constitute what the workmen call "half-stuff." This is mixed with chloride of lime (the machine being again set in motion) for the purpose of bleaching the pulp, in which solution it remains until it is quite colourless. The pulp is now either put into another machine of the same description, which cuts sharper and finer; or else the same machine, used at first, is so screwed up as to cause the knives to come more closely together; in either case, the rate of turning is greatly increased, so that the wheel makes about 150 revolutions per minute, and completely grinds up the pulp till it is. perfectly smooth. At this part of the process various colours are mixed with the pulp, such as ultramarine,

chrome, and the "aniline dyes." These are seen in blue "foolscap,' ," "cream-laid," "toned," and other varieties.

Paper is now nearly all made by machinery, in pieces of any width up to 120 inches, but of an indefinite length, and is afterwards cut up into sheets by a "cutting machine." The paper-making machine consists of a reservoir for the fine pulp, prepared as before described, and fitted with regulating valves for limiting the flow of "stuff." This, largely diluted with water, passes through finely-cut strainers, which stop all knots and portions badly prepared, and it is then delivered to an endless revolving sheet of wire-gauze, with from 3,000 to 5,000 perforations in each square inch-so fine that, although by the aid of a shaking arrangement the water gradually passes through, the fibres are retained on the upper surface, and by the constant agitation and the "felting" quality possessed by the beautifully serrated edges of the fibres, the fluid sheet of pulp is in a few seconds so strong, that it can be carried with but little assistance to the subsequent stages.

The required width of the sheet is given by means of an adjustable strap or "deckle," and the rapid drying is greatly facilitated by boxes with perforated tops, fixed under the travelling wire-cloth. In these a partial vacuum is made by powerful air-pumps, and the pressure of the atmosphere forces the water through the wire into the boxes, leaving the pulp comparatively dry.

Water-marks, such as names, dates, figures, &c., are given in this damp state by means of a light wirecovered roller, called "a dandy." The required pattern is worked on its surface with brass wire, which, penetrating partly through the moist pulp, leaves its impress at each revolution.

The same (6 dandy," according to the way in which it is covered, makes the difference between "laid” and "wove" papers; the former is produced by a covering of parallel wires, fastened at intervals of an inch or so; the latter, by a cover of similar character to the wirecloth on which the pulp travels. The "laid" appearance is well seen in ordinary foolscap.