They found that it was owing to molecular and physical changes caused in it by impurities in the air used and by the high temperature employed for decomposing the dioxide. They discovered that by heating the dioxide in a partial vacuum the temperature necessary to drive off its oxygen was much reduced. They also found that by supplying the air to the baryta under a moderate pressure, its absorption of oxygen was greatly assisted. Under these conditions, and by carefully purifying the air before use, they found that it became possible to use the baryta an indefinite number of times. Thus the process became practically, as it was theoretically, continuous. After securing patent protection for their process, Messrs. Brin erected a small producer in Paris, and successfully worked it for nearly three years without finding a renewal of the original charge of baryta once necessary. This producer was exhibited at the Inventions Exhibition in London, in 1885. Subsequently an English company was formed, and in the autumn of last year Brin's Oxygen Company began operations in Horseferry Road, Westminster, where a large and complete demonstration plant was erected, and the work commenced of developing the production and application of oxygen in the industrial world. [Illustration: APPARATUS FOR MAKING OXYGEN.] We give herewith details of the plant now working at Westminster. It is exceedingly simple. On the left of the side elevation and plan are shown the retorts, on the right is an arrangement of pumps for alternately supplying air under pressure and exhausting the oxygen from the retorts. As is shown in the plan, two sets of apparatus are worked side by side at Westminster, the seventy-two retorts shown in the drawings being divided into two systems of thirty-six. Each system is fed by the two pumps on the corresponding side of the boiler. Each set of retorts consists of six rows of six retorts each, one row above the other. They are heated by a small Wilson's producer, so that the attendant can easily regulate the supply of heat and obtain complete control over the temperature of the retorts. The retorts, A, are made of wrought iron and are about 10 ft long and 8 in. diameter. Experience, however, goes to prove that there is a limit to the diameter of the retorts beyond which the results become less satisfactory. This limit is probably somewhat under 8 in. Each retort is closely packed with baryta in lumps about the size of a walnut. The baryta is a heavy grayish porous substance prepared by carefully igniting the nitrate of barium; and of this each retort having the above dimensions holds about 125 lb. The retorts so charged are closed at each end by a gun metal lid riveted on so as to be air tight. From the center of each lid a bent gun metal pipe, B, connects each retort with the next of its series, so that air introduced into the end retort of any row may pass through the whole series of six retorts.