The demand for bleaching powder is great and steadily increasing. The price of 35 per cent. bleaching powder has never been less than about £5 a ton,[3] so that it is perhaps unnecessary to add that the absorption of hydrogen chloride gas is now made so complete that it is well within the requirements of the second Alkali Act. Chlorides. The salts of hydrochloric acid are called chlorides, and the most important of these is sodium chloride or common salt—a body that is so well known that it need not be described here. Although the quantity of this substance required for domestic purposes is very large, it is, nevertheless, small by comparison with that which is used for industrial purposes. It has already been mentioned that salt is the starting-point for the manufacture of washing soda by the Leblanc process, and, in addition to this, it is employed in the glass industry to produce whiteness and transparency in certain kinds of glass; in pottery, for glazing earthenware; in soap-making, for salting out the crude soap; and in the dye trade as a mordant, and also for improving the quality of certain colours. A full account of the salt industry is given in another volume of this series. Hydrofluoric Acid. When calcium fluoride (fluorspar, Derbyshire spar, or blue-john) is warmed with concentrated sulphuric acid in a leaden dish, hydrogen fluoride gas is evolved, and this, when dissolved in water, gives hydrofluoric acid. The peculiar property of this substance is that it has a very marked corrosive action on glass. It cannot, therefore, be kept in glass vessels, but must be stored in bottles made of hardened caoutchouc. On the other hand, it is this same property which gives it its place in commerce. As far back as 1670 it was used for etching on glass. The process is a very simple one. The article is first coated with wax, which is then removed in places by a sharp pointed tool. When exposed to the action of the gas or its solution, corrosion takes place only where the glass has been laid bare, the other parts being protected by the wax. After a short interval, the wax can be melted off, and the design made more distinct by rubbing in some opaque cement. For general trade purposes, such as the stamping of lamp chimneys or electric light bulbs, a quicker method is required. In this case, a preparation of hydrofluoric acid which can be applied with a rubber stamp is used. Fluorspar or calcium fluoride is the most important salt of hydrofluoric acid. It is a commonly occurring mineral, and besides its use for the preparation of the acid, it is employed in many metallurgical operations to form a fusible slag.