The gases coming from the pans in which the salt was decomposed were led into towers (see Fig. 8) built of bricks or Yorkshire flags soaked in tar. These towers were filled up with coke or other acid-resisting material, which was kept moist by water flowing from the tank F. In this way, hydrogen chloride gas was removed and hydrochloric acid collected in tanks (not shown in the figure) at the bottom of the towers. Even then, there was no market for the greater part of the recovered acid, consequently much of it found its way into drains and streams, and so carried on its work of destruction in a less obtrusive way. [Illustration: Fig. 8. PREPARATION OF HYDROCHLORIC ACID] By another piece of legislation, which at first sight seems to be wholly unconnected with Chemistry, hydrochloric acid acquired a greatly enhanced value. In 1861, the tax on paper was removed, and in the next twenty years the demand for that commodity increased so much that raw material both cheaper and more abundant than rag had to be found. Esparto grass and eventually wood pulp proved successful substitutes. There is really very little difference in composition between cotton and linen rag on the one hand and wood fibre on the other, for both are mainly composed of cellulose, which is a definite chemical compound. Wood fibre is the less pure, and it is also coloured, and therefore has to be bleached before it can be used for making white paper. It was this circumstance which led to the greatly increased demand for hydrochloric acid. At the beginning of this chapter, it was mentioned, in passing, that hydrogen chloride gas is a compound of hydrogen and chlorine. The latter element is a very active bleaching agent, and is most easily obtained by treating hydrogen chloride or its solution in water with pyrolusite (black oxide of manganese), whereby the hydrogen is oxidized, forming water, and chlorine gas is set free. Being a gas, chlorine is not convenient to handle in large quantities; it is, therefore, converted into bleaching powder, commonly but wrongly called chloride of lime. Bleaching Powder. The manufacture of bleaching powder is carried out in the following way. Slaked lime to the depth of 3 or 4 in. is spread over the floor of a special chamber which can be made gas-tight. The lime is raked up into ridge and furrow, and the chamber is filled with chlorine. At the end of about twenty-four hours, the greater part of this chlorine will have been absorbed by the lime. The chamber is then opened, the lime is raked over to expose a fresh surface, and the process of chlorination is repeated. Generally this is sufficient; the bleaching powder should then contain about 35 per cent. of available chlorine.