Schafheutl and Brodie were the first to show that when graphite is suspended in concentrated sulphuric acid, addition of a small quantity of nitric acid causes it to assume a steel-blue lustre in reflected light, whereas in transmitted light it appears a purple colour. Unaware of this reference A. Frenzel, made the same observation. The closer study of this phenomenon showed us that this " blue graphite " is a chemical compound of graphite and sulphuric acid.We, and also H. Thiele,4 found that the graphite-sulphuric acid compound in concentrated sulphuric acid is formed by numerous oxidising agents other than nitric acid, for example, CrO,, KMnO,, (NH,),S,O,, PbO,, MnO,, HIO,, HIO,, Mn3+ and Mn4+ salts, and so on, as well as by anodic oxidation.Analogous compounds can be obtained from graphite and other concentrated acids other than sulphuric acid, upon oxidat ion, and examples of these are perchloric acid, selenic acid, nitric acid, and less easily with phosphoric acid, pyrophosphoric acid, and arsenic acid. All these compounds are stable only in the presence of the concentrated acid, decomposition occurring on addition of water, or even in the presence of atmospheric moisture.* The (hkl) interferences of many crystals require an identity period twice as great ; the (ool) interferences are, however, identical in both cases-also in intensity-so that the parameters of the atoms in the direction of the C-axis must also be identical.