ON SILICA AND BORIC ANHYDRIDE I77 DISC USS ION. Dr. R. Seligman : I am inucli interested in the experiments which Messrs. Weston and Ellis have brought before us, and should like to draw attention to one or two points, I was, unfortunately, not present when Dr. Perkin read his paper comparing the activities of calcium and aluminium, but I understand that my colleague, Mr. 147. Murray-Morrison, suggested that the apparent inactivity of aluminium in reducing silica was due to the coarseness of the ingredients used. The proof of the accuracy of this statement is contaiiied in the present paper, if such proof were needed. I say “if such proof were needed” because a method of controlling the rate of reaction of thermite mixtures by varying the grain size of the ingredients has actually been patented by Beuk.The effect of temperature on the rate of the reaction is neatly brought out in the authors’ paper. The retardation due to the excess of the oxide is also interesting. The case is, of course, not isolated, the same phenomenon being observed, for instance, with tungstic acid, where a considerable excess of aluminium is required in order to make the reaction take placc. That it should be SO marked in the case of boric anhydride is a little surprising, and would lead to the supposition that in the case where A1 and B,03 are mixed in theoretical proportions, the combination of aluminium with air may be responsible for the generation of some of the heat required to keep the reaction going. The authors assume that the boron produced burned again in air, and cite the production of an ammonia-yielding body as evidence of the formation of boron nitride.This evidence is not conclusive, because when aluminium is burned in air aluminium nitride is produced. I think, therefore, that the authors’ observation confirms my view. The fact that the aluminium nitride is not at once decomposed by the acids used by the authors is probably due to its being surrounded by fused alumina. Perhaps I may be allowed a slight digression at this point. I think it is not generally known that at one stage in the preparation of pure aluminium thermitic temperatures (if I may coin the word) are obtained. These tem- peratures are produced solely by the combustion of aluminium in the air, and the results of this combustion on a large scale are rather peculiar.Thus, for instance, we frequently get large masses of fused, or partly fused, alumina which, cooling slowly, often contain nests of acicular crystals, which show the most delicate shades of blue or red according to their content of foreign matter. I had hoped to be able to show you some of these to-night, but have been disappointed in my efforts to get them up from the works in time. There is only one other point which I want to raise, and which is of general interest in connection with all thermitic processes ; I mean the fused alumina which caused the authors so much trouble in the present case. Perhaps you know that a use for this material has now been found, and that several patents have been taken out for preparing fire- and acid-resisting ware from it for which great claims are made.I am able to show you a piece of this material. The rest of thc vessel of which it formed a portion is being tested in Staffordshire. I think that any one who has had to deal with high temperatures, whether on a large or small scale, must view the advent of :i new fire-resisting medium with the greatest interest. Dr. F. M. Perkin said he had tried the kieselguhr reaction, but had used molecular proportions and not varying proportions as the authors had done. Under the conditions he had employed the reaction worked very badly.178 NOTE ON THE ACTION OF ALUMINIUM POWDER The great difficulty in employing this I‘ thermitic ” reaction on a laboratory scale was to get rid of the oxide, because, generally speaking, when carried out on a small scale it did not fuse or form a slag.The careful work done by the authors in getting rid of the oxides chemically after the reaction was complete was very interesting, but one could not say that the problem was solved until pure silicon and pure boron had been obtained. Upon a small scale either with aluminium or calcium this had not been done. It was doubtful even whether absolutely pure boron had ever been produced. Mr. F. E. Weston in reply, said that with regard to the question of the burning of the aluminium which had been raised, molecular mixtures could be made which would not start with a fuse. Possibly in such cases some preliminary burning of aluminium gave the temperature required to start the reaction. However, in many experiments they had made, the air had been excluded entirely from the mixture, which, when treated in a muffle, reacted quite as well as if open to the air. He thought that once the action had started, the air entered no further into the matter. The question of the necessity for the slag fusing before the metals could be reduced in reguline form had been raised. The authors had made metallic nuggets of iron, chromium, and manganese on a small scale, but had not succeeded in preparing boron, silicon, titanium, and tungsten, excepting in the form of powder.