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Correspondence
EXHIBIT AVAILABLE ON MANUFACTURE OF PORTLAND CEMENT To the Editor DEARSIR: From time to time we receive requests from teachers, students, and others asking for educational material on the manufacture of cement. At the present time we have no literature on the manufacture of cement which we can furnish to schools, teachers, and others. We can, however, if desired, furnish an exhibit set which consists of eight display bottles containing samples of both raw and finished products used in the manufacture of portland cement. We shall be glad to furnish these exhibit sets on request a t 50 cents per set. This nominal charge is made t o cover the cost of the containers and postage. Yours very truly, M. A. BERNS,Publicity Manager UNIVERSAL A n ~ CEMENT s CO. CHICAGO, ILLINOIS
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AN IONIC EXPLANATION OF THE SEPARATION OF THE CATIONS OF THE TIN SUBGROUP WITH THE REAGENT SODIUM HYDROXIDE To the Editor DEARSIR: It is common practice in qualitative analytical procedure to make'a separation of the copper and tin subgroups in ammonium sulfide, ammonium polysdiide, sodium sulfide, or sodium hydroxide, leaving undissolved the more basic cations which comprise the so-called copper subgroup. This letter is concerned with the use of sodium hydroxide as the dissolving reagent and more particularly with the ionic explanation of the reaction. The current manuals of qualitative analysis give a perfectly satisfactory explanation for the dissolving of the tin subgroup by sodium and ammonium sulfides hut leave one very much in the dark as to just why and how these sulfides dissolve in sodium hydroxide. The writer wishes therefore to propose an explanation in harmony with the analytical situation and with the ionic theory, making, however, no claim to originality but hoping to 1486
