The sulphuric acid so formed, together with the excess of condensed steam, collect upon the floor of the chambers in the form of a liquid containing from 62% to 70% of sulphuric acid. The product is called _chamber acid_ and is quite impure; but for many purposes, such as the manufacture of fertilizers, it needs no further treatment. It can be concentrated by boiling it in vessels made of iron or platinum, which resist the action of the acid, nearly all the water boiling off. Pure concentrated acid can be made best by the contact process, while the chamber process is cheaper for the dilute impure acid. ~Physical properties.~ Sulphuric acid is a colorless, oily liquid, nearly twice as heavy as water. The ordinary concentrated acid contains about 2% of water, has a density of 1.84, and boils at 338 deg.. It is sometimes called _oil of vitriol_, since it was formerly made by distilling a substance called _green vitriol_. ~Chemical properties.~ Sulphuric acid possesses chemical properties which make it one of the most important of chemical substances. 1. _Action as an acid._ In dilute solution sulphuric acid acts as any other acid, forming salts with oxides and hydroxides. 2. _Action as an oxidizing agent._ Sulphuric acid contains a large percentage of oxygen and is, like nitric acid, a very good oxidizing agent. When the concentrated acid is heated with sulphur, carbon, and many other substances, oxidation takes place, the sulphuric acid decomposing according to the equation H_{2}SO_{4} = H_{2}SO_{3} + O. 3. _Action on metals._ In dilute solution sulphuric acid acts upon many metals, such as zinc, forming a sulphate and liberating hydrogen. When the concentrated acid is employed the hydrogen set free is oxidized by a new portion of the acid, with the liberation of sulphur dioxide. With copper the reactions are expressed by the equations (1) Cu + H_{2}SO_{4} = CuSO_{4} + 2H, (2) H_{2}SO_{4} + 2H = H_{2}SO_{3} + H_{2}O, (3) H_{2}SO_{3} = H_{2}O + SO_{2}. By combining these equations the following one is obtained: Cu + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O. 4. _Action on salts._ We have repeatedly seen that an acid of high boiling point heated with the salt of some acid of lower boiling point will drive out the low boiling acid. The boiling point of sulphuric acid (338 deg.) is higher than that of almost any common acid; hence it is used largely in the preparation of other acids. 5. _Action on water._ Concentrated sulphuric acid has a very great affinity for water, and is therefore an effective dehydrating agent. Gases which have no chemical action upon sulphuric acid can be freed from water vapor by bubbling them through the strong acid. When the acid is diluted with water much heat is set free, and care must be taken to keep the liquid thoroughly stirred during the mixing, and to pour the acid into the water,--never the reverse.