phenomenon we have rhythmic precipitation which depends only on forces intrinsic to the system, and is independent of any corôÙreôÙsponôÙding rhythmic changes in temperature or other external conditions, we have not far to seek for instances of chemico-physical phenomena where rhythmic alternations of appearance or structure are produced in close relation to periodic fluctuations of temperature. A well-known instance is that of the Stassfurt deposits, where the rock-salt alternates regularly with thin layers of ãanhydrite,ã or (in another series of beds) with ãpolyhalite44ã: and where these zones are commonly regarded as marking years, and their alternate bands as having been formed in connection with the seasons. A discussion, however, of this remarkable and significant phenomenon, and of how the chemist explains it, by help of the ãphase-rule,ã in connection with temperature conditions, would lead us far beyond our scopeÿ£¢45.
We now see that the methods by which we attempt to study the chemical or chemico-physical phenomena which accompany the development of an inorganic concretion or spicule within the {434} body of an organism soon introduce us to a multitude of kindred phenomena, of which our knowledge is still scanty, and which we must not attempt to discuss at greater length. As regards our main point, namely the formation of spicules and other elementary skeletal forms, we have seen that certain of them may be safely ascribed to simple precipitation or cryôÙstalôÙliôÙsaôÙtion of inorganic materials, in ways more or less modified by the presence of albuminous or other colloid substances. The effect of these latter is found to be much greater in the case of some crystallisable bodies than in others. For instance, Harting, and Rainey also, found as a rule that calcium oxalate was much less affected by a colloid medium than was calcium carbonate; it shewed in their hands no tendency to form rounded concretions or ãcalôÙcoôÙspheôÙritesã in presence of a colloid, but continued to crystallise, either normally, or with a tendency to form needles or raphides. It is doubtless for this reason that, as we have seen, crystals of calcium oxalate are so common in the tissues of plants, while those of other calcium salts are rare. But true calôÙcoôÙspheôÙrites, or spherocrystals, of the oxalate are occasionally found, for instance in certain Cacti, and Bû¥tschli46 has succeeded in making them artificially in Hartingãs usual way, that is to say by cryôÙstalôÙliôÙsaôÙtion in a colloid medium.
There link on to these latter observations, and to the statement already quoted that calcareous deposits are associated with the dead products