This was formerly called “Oil of Vitriol,” because it has an oily consistence, and was originally distilled from green vitriol. Sulphuric acid is one of the most useful chemical agents known; scarcely a process in chemical manufacture can be performed without its assistance. Sulphuric acid is colorless, and very heavy, being nearly double that of water (the proportion is 1·842, while water is 1,000); it is powerfully acid, even when largely diluted with water, and during this mixture with water gives out great heat, a mixture of equal parts will become hotter than boiling water; if it be mixed, however, with snow instead of water, it becomes extremely cold (below zero). Sulphuric acid is made by burning sulphur in a furnace, and causing the vapours to pass into a large chamber lined with lead, and having some water at the bottom; into this chamber the vapours arising from nitre mixed with sulphuric acid are also admitted, together with air and a jet of steam. These arrangements require to be properly and nicely regulated, and it was only by study of the complicated changes which take place between these gases (sulphurous acid, nitric oxide, and atmospheric air), that this arrangement has been devised and adopted, a great part of the product having been formerly wasted in a more clumsy mode of preparation. The sulphuric acid condensed in the water at the bottom of the leaden chamber is too weak for use, and is concentrated by evaporating the water from it; for this purpose it is placed in shallow leaden pans placed on the bars of a furnace, and finally distilled in glass retorts, or retorts of iron lined with platinum. Sulphuric acid has such a powerful attraction for water, that an open vessel half-full of strong acid placed in a damp situation will attract enough water from the atmosphere to cause it to be quite full before long. This power of attracting water has been taken advantage of to procure ice in those places where it is not to be had naturally, as India, &c. If a vessel of water be placed under the receiver of an air-pump, and the receiver exhausted of air, the vapour of the water will speedily fill it, taking the place of the air, and so stop any further evaporation; but if another vessel containing some sulphuric acid be placed also in the receiver, the acid will absorb the vapour of water as fast as it is formed, and this rapid evaporation continuing produces such cold, that the remaining water is shortly frozen. Iron and zinc dissolve rapidly in diluted sulphuric acid, giving off abundance of hydrogen gas, and this was the way this gas was formerly produced for the inflation of balloons, but the common coal-gas being easily obtainable in almost any quantity in all towns, it is now used for that purpose instead of hydrogen gas. Sulphuric acid (chemically considered) consists of 1 equivalent of sulphur with 3 of oxygen, and 1 of water. Pure sulphuric acid (without water) is in the form of fine crystals, much