_b._ OCCURRENCE AND EXAMINATION OF LICHEN-ACIDS. Acids have been found, with few exceptions, in all the lichens examined. They are sometimes brightly coloured and are then easily visible under the microscope. Generally their presence can only be determined by reagents. Over 140 different kinds have been recognized and their formulae determined, though many are still imperfectly known. As a rule related lichen species contain the same acids, though in not a few cases one species may contain several different kinds. In growing lichens, they form 1 to 8 per cent. of the dry weight, and as they are practically, while unchanged, insoluble in water, they are not liable to be washed out by rain, snow or floods. Their production seems to depend largely on the presence of oxygen, as they are always found in greatest abundance on the more freely aerated parts of the thallus, such as the soredial hyphae, the outer rind or the loose medullary filaments. They are also often deposited on the exposed disc of the apothecium, on the tips of the paraphyses, and on the wall lining the pycnidia. They are absent from the thallus of the Collemaceae, these being extremely gelatinous lichens in which there can be little contact of the hyphae with the atmosphere. No free acids, so far as is known, are contained in _Sticta fuliginosa_, but a compound substance, trimethylamin, is present in the thallus of that lichen. It has also been affirmed that acids do not occur in any _Peltigera_ nor in two species of _Nephromium_, but Zopf[818] has extracted a substance peltigerin both from species of _Peltigera_ and from the section _Peltidea_. For purposes of careful examination freshly gathered lichens are most serviceable, as the acids alter in herbarium or stored specimens. It is well, when possible, to use a fairly large bulk of material, as the acids are often present in small quantities. The lichens should be dried at a temperature not above 40°C. for fear of changing the character of the contained substances, and they should then be finely powdered. When only a small quantity of material is available, it has been recommended that reagents should be applied and the effect watched under the microscope with a low power magnification. This method is also of great service in determining the exact position of the acids in the thallus. In micro-chemical examination, Senft[819] deprecates the use of chloroform, ether, etc., seeing that their too rapid evaporation leaves either an amorphous or crystalline mass of material which does not lend itself to further examination. He recommends as more serviceable some oil solution, preferably “bone oil” (neat’s-foot oil), in which a section of the thallus should be broken up under a cover-glass and subjected to a process of slow heating; some days must elapse before the extraction is complete. The surplus oil is then to be drained off, the section further bruised and the substance examined.