SPONTANEOUS COAGULATION.--All readers will be aware of the phenomenon of the curdling or souring of milk. The behaviour of _Hevea_ latex, under certain conditions, may be taken to be analogous. Difficulty is experienced in maintaining fluidity--a difficulty which appears to vary in great degree according to locality, nature of soil, age of trees, the relative demand made upon the trees by the system of tapping employed, etc. It is sometimes found, before the latex reaches the store, that it may exhibit one of various stages of premature (spontaneous) coagulation: (_a_) To all appearances it may be quite fluid, but a close examination shows it to consist mainly of a serum containing very minute particles of rubber in suspension (microscopic coagulation). (_b_) In a later stage these particles coalesce to form larger "flocks" (macroscopic coagulation). (_c_) The whole, or practically the whole, of the latex may have coagulated, forming one mass of rubber with a milky residual serum. Passing from this aspect of the question, it may be noted as peculiar facts that: (1) A shallow layer of latex is less likely to coagulate spontaneously (_i.e._, without the addition of a coagulant) than a deeper volume. (2) The shallow layer, and also the surface of the deeper volume (where exposed to air), on standing will be found to develop a superficial film of finely coagulated particles, yellowish in colour, and having an offensive odour due to decomposition of protein matter. (3) While this partial coagulation is confined only to the surface of a shallow layer of latex, it will be found that below the surface film of the deeper volume a much more definite coagulation has taken place. The coagulation will be practically complete, and the coagulum, apart from a spongy appearance, is normal in character. This coagulum is free from the offensive odour noted above. (4) On testing the surface film of both the shallow layer and the deeper volume, it will be found to be _alkaline_ in character; whilst the lower liquid surrounding the main portion of the coagulum in the deeper volume of latex is of an _acid_ nature. These observed facts are sufficient to indicate that there are apparently _two distinct types of spontaneous coagulation_, and that the latter takes place particularly where the latex is more or less out of contact with the atmosphere. We may, therefore, differentiate thus: (_a_) _In contact with air (aerobic)_: incomplete spontaneous coagulation, accompanied by yellowish slime, offensive in odour and alkaline in character. (_b_) _Out of contact with air (Anaerobic)_: Practically or wholly complete. There is no offensive odour under normal conditions and the serum is acid in character.