The use of acids for distinguishing certain rock-forming minerals is older than the use of the microscope for this purpose. It has long been known that nepheline and a few other silicates are readily decomposed by acids, and that gelatinous silica is formed by the reaction. H. Behrens seems to have been the first to suggest the idea of making the silica gel visible by impregnating it with dyestuff, but he gave no detailed account of his procedure. E. Boricky, who devised many ingenious methods of microchemical analysis, used chlorine gas to decompose the silicates and stained the resulting silica gel with fuchsine solution. All the soda-felds-pathoids and even olivine could be stained in this way, but not leucite. F. Zirkel describes briefly how the thin section is to be covered with a thin film of hydrochloric acid and then, after washing, immersed in a weak aqueous solution of fuchsine or methyl violet. Similar directions are given in other text-books, but often the instructions are so vague that one wonders whether the author ever practised the method himself or just related it at second hand. For instance, E. Weinschenk says the section that has been gelatinized by acid should be immersed “some hours” in congo-red or malachite green; yet anybody who has ever stained feldspathoids must know that it is not necessary to leave the preparation in the dye for more than a minute. Nobody seems to have given much attention to the effects produced by acids other than hydrochloric, or to the method of preserving the stained section as a permanent preparation. In order to make the fullest use of the staining method, these points deserve attention.