The size of the chambers has varied a great deal. In the early years of the nineteenth century, the capacity of a single chamber was probably not more than 1,000 cu. ft.; at the present time, 38,000 cu. ft. is an average size, and there may be three or five of these chambers. The necessity for this large amount of cubic space is easily accounted for. The reaction materials are all gases, and a gas occupies more than one thousand times as much space as an equal weight of a solid or liquid. Moreover, oxygen constitutes only about one-fifth of the total volume of air used in burning the pyrites; the other four-fifths is mainly nitrogen, which, though it does not enter into the reaction at all, has to pass through the chambers. Modern Improvements. Among the modern innovations in the lead chamber process, the following are worthy of note. “Atomized water,” that is, water under high pressure delivered from a fine jet against a metal plate, has certain advantages over steam. In order to bring about a more rapid mixing of the gases in the chamber, it is proposed to make these circular instead of rectangular, and to deliver the gases tangentially to the sides. Another suggestion is to replace the lead chambers by towers containing perforated stoneware plates set horizontally. By this arrangement, since the holes are not placed opposite one another, the gases passing up the tower must take a zig-zag course. This makes for more efficient mixing. THE CONTACT PROCESS Sulphur Trioxide. When elements are combined in different proportions by weight, they produce different compounds. Thus, in the case of sulphur and oxygen, there are two well-known compounds, namely, sulphur dioxide and sulphur trioxide. In the former, a given weight of oxygen is combined with an _equal_ weight of sulphur; in the latter, this same weight of sulphur is combined with 50 per cent. more oxygen. On this account, sulphur trioxide is spoken of as the higher oxide. We can now state in general terms another method by which sulphuric acid can be built up from its elements. Sulphur, as we have seen, burns in oxygen, forming sulphur dioxide. This substance can then be made to unite with more oxygen to give sulphur trioxide, which, with water, yields sulphuric acid. There are three steps in this synthesis. The first, namely, sulphur to sulphur dioxide, has already been considered; the last, sulphur trioxide to sulphuric acid, only requires that sulphur trioxide and water shall be brought together: we can, therefore, confine our attention to the intermediate step, namely, the conversion of sulphur dioxide into trioxide.