There are, in addition, a short series of coloured substances which are of uncertain position. They are imperfectly known and are of rare occurrence. An acid containing nitrogen has been extracted from _Roccella fuciformis_, and named picroroccellin[831] (C₂₇H₂₉N₃O₅). It crystallizes in comparatively large prisms, has an exceedingly bitter taste, and is very sparingly soluble. It is the only lichen-acid in which nitrogen has been detected. One acid at least, belonging to the Fat series, vulpinic acid, which gives the greenish-yellow colour to _Letharia vulpina_, has been prepared synthetically by Volkard[832]. F. CHEMICAL REAGENTS AS TESTS FOR LICHENS The employment of chemical reagents as colour tests in the determination of lichen species was recommended by Nylander[833] in a paper published by him in 1866. Many acids had already been extracted and examined, and as they were proved to be constant in the different species where they occurred, he perceived their systematic importance. As an example of the new tests, he cited the use of hypochlorite of lime, a solution of which, applied directly to the thallus of species of _Roccella_, produced a bright-red “erythrinic” reaction. Caustic potash was also found to be of service in demonstrating the presence of parietin in lichens by a beautiful purple stain. Many lichenologists eagerly adopted the new method, as a sure and ready means of distinguishing doubtful species; but others have rejected the tests as unnecessary and not always to be relied on, seeing that the acids are not always produced in sufficient abundance to give the desired reaction, and that they tend to alter in time. The reagents most commonly in use are caustic potash, generally indicated by K; hypochlorite of calcium or bleaching powder by CaCl; and a solution of iodine by I. The sign + signifies a colour reaction, while- indicates that no change has followed the application of the test solution. Double signs ⁺₊ or any similar variation indicate the upper or lower parts of the thallus affected by the reagent. In some instances the reaction only follows after the employment of two reagents represented thus: K(CaCl)+. In such a case the potash breaks up the particular acid and compounds are formed which become red, orange, etc., on the subsequent application of hypochlorite of lime. As an instance of the value of chemical tests, Zopf cites the reaction of hypochlorite of lime on the thallus of four different species of _Gyrophora_, the “tripe de roche”:— Gyrophora torrefacta CaCl ⁻₊. ” polyrhiza CaCl ⁺₊. ” proboscidea CaCl ⁺₋. ” erosa CaCl ⁻₋. It must however be borne in mind that these species are well differentiated and can be recognized, without difficulty, by their morphological characters. Experienced systematists like Weddell refuse to accept the tests unless they are supported by true morphological distinctions, as the reactions are not sufficiently constant.