Page:Hardwicke's Science-Gossip - Volume 1.pdf/68

52 CLEANING DIATOMACEÆ.

T is often a source of difficulty to a microscopist who for the first time attempts the study of the various forms of Diatomaceæ contained in infusorial earths and guano to entirely free them from foreign matters. In order to do this some knowledge of chemistry is requisite, more, indeed, than is often possessed by microscopists; it is, therefore, the object of the writer in the following remarks to supply this to a certain extent by pointing out some of the constituents of these deposits and guanos, which are the principal sources of our foreign and some of our fossil Diatomaceæ.

The shells of the Diatomaceæ, it is well known, are composed of silex, or, in common parlance, flint, and are consequently not acted on by any of the ordinary mineral acids in common use, viz., the nitric, hydrochloric, or sulphuric; but, on the other hand, boiling in solution of the caustic alkalies, soda and potash, will dissolve them. It is also to be observed, that they are unalterable by the action of a red heat, but if heated with either soda or potash, the silex combines with the alkali, and fusion, and consequent loss, takes place.

These facts are well to be noted, that the operator may know what chemical agents are to be avoided in working on substances containing these minute organisms.

With regard to the constituents of the earths in which the shells are found, these consist principally of lime in various states of combination, usually either carbonate or sulphate, or perhaps both, frequently coloured with oxide of iron, and also silex in the form of sand, which latter can only be separated by careful fractional washing.

The carbonate of lime is soluble with effervescence in nitric or hydrochloric acid; either of these will form a soluble salt which may easily be removed by washing. Sulphuric acid should be avoided, for it forms with the lime a sparingly soluble sulphate. Sulphate of lime, if present as a constituent, will probably not be in very large quantity, and may be removed by frequent washing, as it is soluble to a certain extent in water. The oxide of iron, also small in quantity, may be dissolved by the nitric or hydrochloric acid, and should these fail when used separately (which will sometimes be the case from the peculiar state of combination in which the iron exists), they may be employed mixed together, forming aqua regia. The process of cleaning may be known to be completed when the deposit is quite white and acid have no further action on it; a small portion should then be examined (under water) by the microscope, and if quite clean should be well washed, first with common water, and finally with distilled water. The latter is indispensable to complete the operation, that all traces of acid and soluble salts may be removed.

If the earth under examination occurs in tolerable sized masses it should be broken into fragments about as large as a small pea, and then dropped gradually into a Florence flask or open evaporating basin, about half full of nitric or hydrochloric acid, diluted with one-third of its volume of distilled water. It is better to drop the substance under operation into the acid than to pour the acids on the material, as the action takes place more gradually and with less frothing than is generally the case when much carbonate is present, which sometimes causes the liquid to escape from the vessels, and consequent loss. When the acid ceases to act, heat may be applied by any suitable means, and when all action ceases the mixture should be allowed to rest, that the deposit may subside, and the liquid poured off as close as possible, and replaced by strong acid, and the acid again aided by gentle heat. Should the deposit not be quite white the mixed acids must be employed, and the process finished by washing with distilled water.

To proceed to guanos; these present more difficulty, their composition being much more complex, and containing, besides the lime salts and oxide of iron above mentioned, salts of soda, potash, and ammonia; also much anima matter, and some phosphate of lime. The salts of the alkalies are soluble in water, and the guano being usually in a more or less disintegrated state, it should be first well washed with boiling water, either on a paper filter or by repeated affusions, until the liquid comes off tasteless. Boiling water is preferable to cold, for the heat expels air-bubbles and causes the deposit to settle down into a smaller space; then the deposit must be subjected to the action of the acids as directed for the preparation of earths, to get rid of the lime salts. The partially cleaned deposit, which is now much smaller in bulk, must be separated as much as possible from the liquid, and strong sulphuric acid, in sufficient quantity to cover it, poured on to about the depth of half-an-inch, heat applied and contained for some time, but the liquid must not be made to boil; the result will be an almost immediate blackening of the liquid,