In all operations carried out on a large scale, the regulation of temperature is a matter of some difficulty. In the case which we are considering, the most suitable temperature range is a rather narrow one, and the difficulty of keeping within the limits is very much increased by the large amount of heat given out when the sulphur dioxide and oxygen combine. The result of the failure to maintain the temperature at a fairly constant level was that the process worked in a very irregular manner, for as soon as it was working really well and sulphur trioxide was being formed rapidly, the heat given out by the reaction itself was also great, and consequently, the higher temperature limit was exceeded. The method of controlling the temperature in the contact process is worth noting, because it is really ingenious. The tubes containing the platinized asbestos are surrounded by wider concentric tubes. The gases which are about to enter the contact furnace pass through the annular space between the two tubes, and are thereby heated to the required temperature, while at the same time they serve to cool the inner tubes. The most satisfactory temperature is about 400° C. The tubes are first warmed to 300° C. to start the reaction, and thereafter the heat evolved by the reaction itself is sufficient to keep it going. The absorption of the sulphur trioxide also caused some difficulty at first. This substance reacts most violently with water, dissolving with a hissing sound like that produced when a red-hot poker is plunged into water. At the same time great heat is developed, and consequently, much of the sulphur trioxide is vaporized, and in that way lost. This difficulty was got over by using 98 per cent. sulphuric acid for the absorption, the acid being kept at this strength by the simultaneous addition of water. The contact process has some very distinct advantages over the older lead chamber process. The plant covers a much smaller area than the bulky lead chambers. Although the preliminary purification of the gases is somewhat tedious and costly, this is in great measure compensated by the purity of the acid produced. No separate plant is required for concentration and purification, as in the older process. Finally, sulphuric acid of any concentration can be produced at will, including the fuming acid, which is required as a solvent for indigo, and in the manufacture of artificial indigo and other organic chemicals.