(3) The process here described consists in corroding glass by violently projecting sand upon its surface by means of a current of air or steam. It is very probable that it will be found of service in a great variety of altered at will, so as to regulate the fall of the sand. The tube conveying the current of air or steam terminates just above this spout, in a nozzle containing a series of fine holes. The sand, urged on by the jet, is thrown violently against the glass plate e, or other body placed within its range, and thus exerts a corroding action. By varying the quantity of the sand, the volume and velocity of the current, as well as the diameter of the jet, more or less rapid effects are produced.

Etching glass.

Etching glass.

Bodies much harder than glass have been submitted to the action of sand thus thrown forcibly against their surface, and have been as rapidly worn away. In a series of experiments recently conducted in New York, a hole 3 cm. diameter was drilled through a block of corundum in 25 minutes, a pressure of 300 lb. being used. With a pressure of 100 lb., a hole 3 cm. diameter and 8 mm. depth was formed in a steel file in 3 minutes. The weight of a diamond was sensibly diminished in a minute, and a topaz utterly destroyed.

In engraving on glass very little pressure is needed, the current from the bellows of an enameller's lamp being quite sufficient. In this way the divisious on graduated tubes, the labels on bottles, etc, can easily be engraved in laboratories with but little trouble.

The portions of the glass which are to remain clear are covered with paper, or with an elastic varnish, these substances being sufficiently exempt from the corroding action of the sand.

(4) A. Miiller Jacobs has .lately described a photomechanical process for , etching on glass which possesses several ' novel features. The inventor has previously shown that certain resinate colours are sensitive to light, and, after exposure to light, the dye stuff becomes soluble in alcohol or other solvent. The sensitive resinate is made as follows: -

Colophony . 100 grm. Caustic soda . 10 „ Crystals sodium carbonate . 33 „

Water... 1000 c.c;

The mixture is boiled for 2 hours with 1000 c.c. water, and is then mixed with 500 c.c of a hot solution of 7.5 grm. methyl violet 3 B (methyl green, chysoldine, magenta, &c); 60 grm. magnesium sulphate are now gradually stirred into the solution, and the precipitated colour is washed and dried at 60° C. The sensitive film is prepared from these resinate colours by dissolving a mixture of 20 grm. resinate violet, 8 grm. resinate green, 8 grm. of the chysoldine, and 4 grm. of the magenta in 130 c.c. pure benzene and 70 c.c. chloroform. This solution is then mixed with 120 cc. of a caoutchouc solution (50 grm. caoutchouc, digested with 4000 grm. carbon bisulphide, heated on a water bath until half the bisulphide has distilled off, then benzene added to make the total weight 3333 grm.) After standing for a few days the solution is filtered through cotton wool, and kept in the dark for use. The plates can be either of metal or glass, and are coated with this mixture, dried, and exposed to light under the negative which is to be reproduced. The time of exposure varies from J to 3 hours, depending on the intensity of light and the relative amounts of green and red dyes tuffs in the resinate colour used. The exposed plate is kept in a cool, dark place until ready for developing.

This process consists in immersing the plate in a solution of 1 part benzene and 3 parts turpentine. After the solution of the soluble colours, the plate is washed in petroleum spirit and made ready for the etching process. For matt etching on glass the author recommends fuming hydrofluoric acid containing 10 per cent, of water. (Industries,)

(5) Comparatively cheap etching solutions can be prepared, which are equal in effect to the expensive fluorine salts.

I. Two solutions are first prepared, (a) consisting of 10 grm. soda in 20 grm. warm water, (6) consisting of 10 grm. potassium carbonate in 20 grm. warm water. Solutions (a) and (6) are now mixed, and to the-mixture is added 20 grm. concentrated hydrofluoric acid, and afterwards a solution (c) consisting of 10 grm. potassium sulphate in 10 grm. water is added.

II. Mix 4 cc. water, l 1/3 grm. potassium carbonate, 0.5 c.c. dilute hydrofluoric acid, 0.5 c.c. hydrochloric acid, and 0.5 cc. potassium sulphate. This, mixture is treated with hydrofluoric acid and carbonate of potassium until it produces the required degree of opacity on being tried upon a piece of glass, ;

The addition of a small quantity of hydrofluoric acid to solution I. brings about a fine granulated appearance on the surface. (Lainer.)

(6) A still simpler process than either of these has been invented by Kampmann, In preparing an opaque etching fluid Kampmann uses a wooden vessel, the iron fittings of which are protected from the corrosive action of the acid fumes by a layer of as-phaltous material. This vessel is filled to about 1/5 its contents with strong hydrofluoric acid, which is then partially neutralised by cautiously and gradually adding some crystals of soda; more soda is added, and the mixture is stirred with a small wooden rod. The point at which the neutralisation of the acid should cease is indicated by the mixture frothing and becoming sufficiently viscid to adhere to the stirring rod. It is*, perhaps, scarcely necessary to say that the acid fumes are highly injurious, and that this process should be carried on in the open air, in order to allow the vapour to„ pass rapidly away. The most hygienic and satisfactory process of all would be to carry on the operation in a "draught cupboard."

The contents of this wooden vessel now consist of sodium fluoride and the unneutralised hydrofluoric acid. This mixture is transferred to a wooden tub and diluted with 5-10 times its volume of water, according to the degree of dilution desired. It is objectionable to use the mixture in a too highly concentrated condition, for then the etched surface of the glass is irregular, coarsegrained, and apparently strewn with tiny crystals; if, on the other hand, the dilution is top extreme, the etched surfaces will be transparent instead of opaque. Either of these two conditions of the etching fluid can easily be remedied, for if it be too strong water must be added, and if too weak, a small quantity of hydrofluoric acid partially neutralised with soda. A good recipe for preparing a small quantity of this etching fluid is the following; 240 a.c, commercial hydrofluoric acid, 300 grin. powdered crystallised soda, 100 c.c.' water.

These etching fluids are best used by taking the following precautions. The glass is first thoroughly cleansed from all impurities, and is then provided with a rim of wax composed of the following ingredient: - Beeswax, tallow, colophony, and powdered asphalte, kneaded together. The rim prevents the acid from spreading over those parts of the surface which it is not desired to etch. The glass is then etched for a few minutes with an ordinary etching solution (H.F. - 1: 10) which is then poured off, the surface being afterwards washed with water and wiped as dry as possible with a piece of sponge. The surface is then ready for the opaque etching fluid, which is poured on till it forms a thick layer. The operation is allowed to progress for one hour, when -the liquid is poured away and the surface washed with water. Water is further allowed to stand on the glass until a thin film of silicate is observed to form; the film is then brushed off, the surface is finally cleansed with water, and the wax is removed.

By varying the action of this opaque etching fluid or paste, various degrees of opacity may be produced, and if the opacity be greater than that which is desired, the surface can be cleared to any extent by using the etching solution of hydrofluoric acid.