it affords of increasing or decreasing the temperature at pleasure. Care must be taken to prevent the heat becoming high enough to melt the wax.

After being exposed to the acid fumes for a few minutes, the glass plate may be removed and cleaned. The lines where the wax had been removed are found to be covered with a white powder, which consists of silico-fluorides of the metallic bases of the glass. The greater part of the wax may be removed by scraping with a common table-knife, and the remainder by warming the glass before the fire and wiping it with tow and a little oil of turpentine. The design will then be found perfectly etched upon the surface of the glass, the depth of the lines being proportional to the time the glass was exposed to the acid vapour. Glass plates engraven in this manner are not adapted for printing upon paper, like a copper plate, from the facility with which they are broken by the pressure necessary to apply in printing.

In conducting this operation, care should be taken not to allow the hands to become exposed to the acid fumes, as the vitality of the parts would be instantly destroyed by the action of the acid. A dilute aqueous solution of hydrofluoric acid may be substituted for the vapour in the above process, with a similar result.

Glass grinding and cutting.-The implements employed in grinding and cutting glass are extremely simple, though necessarily very varied, owing to the great variety of work which has to be executed. The most important of the instruments employed in

this art are circular discs, seldom exceeding twelve inches in diameter, which are caused to work with great celerity on a horizontal axis, set in motion, in all large establishments, by steam or water power. Against the circumference of these revolving discs, the glass to be ground is held by the hands of the workman.

The thickness of the cutting discs and the forms of their edge are varied considerably; some being flat, others concave, others convex, and others wedgeshaped. Even forty or fifty discs with differentshaped edges may be found in the same workshop. Materials of very different degrees of hardness, from cork to wrought iron, are employed in the formation of these discs. Those made of wrought iron, which are very thin, are used to cut grooves in glass, by the aid of sand and water, which are caused to drop on the circumference of the disc from hoppers placed above. Cast-iron discs are also sometimes used in the roughest part of the operation, but the coarse work is usually done by a disc of fine sandstone wetted with water. When ground down to the proper shape, the glass is polished by exposure to softer discs, the action of which is generally assisted by various powders applied in a humid state to the circumference. Thus a copper disc is used with emery and oil; a disc of block-tin with peroxide of tin or tutty; a disc made of willow wood with fine pumice-stone, colcothar, or putty; and a cork disc having an edging of hat-felt, with putty or colcothar. For grinding down and polishing optical glasses, brass moulds of the required shape are employed, the grinding and polishing powders being,

1o, coarse emery; 2°, fine emery; 3°, fine pumicestone, and 4°, finely washed colcothar, each being wetted with water. The polishing is sometimes completed by a mould of lead. The formation of optical glasses requires some dexterous and delicate manipulations, but the whole art of glass-grinding and cutting is too strictly mechanical to require any further observations on it in a work treating merely of the preparation and chemical history of this highly important and interesting article.

97

STARCH.

§ I. Properties and composition of Starch.-II. Preparation of Starch from Cereals.-III. Preparation of Starch from Rice.-IV. Preparation of Starch from Potatoes.-V. Arrow-root; Sago; Tapioca; Salep; Tous les mois, and Indian Corn Starch.-VI. Dextrin and Starch Sugar.

Ir a piece of dough made of wheaten flour and cold water is tied up in a linen bag and kneaded in cold water, a white pulverulent substance exudes, which is deposited on allowing the liquor to repose. When water no longer passes through the bag in a milky state, a dirty white or greyish substance remains in the bag, differing widely in properties from the separated powder, being characterized while moist by an extraordinary viscidity and adhesiveness. In this way the two essential constituents of flour are separated from each other: the viscid substance remaining in the bag is called gluten, and the white powder separated by washing is starch.

Starch is not a constituent of grains of wheat or the different cereals alone with the exception of woody fibre or lignin, it is probably the most abundantly diffused of all proximate vegetable principles. It is contained in the seeds of all acotyledonous plants; in several round perennial roots which produce an annual stem; in tuberous roots, such as the potatoe; in the stems of many monocotyledonous

VOL. II.

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