In gathering diatoms much labour is saved by judgment and care; hence it is desirable to get acquainted with them in their growing condition, so that when recognised upon the sands or other spots they may be carefully removed by the aid of the spoon or small tin scoop before described. When growing upon algæ or other plants, the plants and diatoms together may be carried home, in which case they must be simply drained and not washed or pressed, in order that the diatoms be not lost. As it is always desirable to examine the gathering on the ground, a “Gairdner’s hand microscope” with powers from 80 to 200 diameters will be found very useful. The best gatherings are those which represent one species abundantly. Those which are mixed may be rejected, unless they are seen to contain something valuable or important, as the object should be not so much to supply microscopical curiosities as to collect material which is available for the study of nature. The gathering when carried home should always be carefully examined before anything is done with it; not only on account of the additional information thus acquired, but also because it often happens that a specimen should be mounted in fluid (see Chapter IV.) in the condition in which it is gathered, as well as cleaned and mounted in balsam (Chapter III.) and dry. Where the gathering is taken from sand, the whole may be shaken up in water as a preliminary operation, when much of the sand will be separated by its own weight. The lime test, however, should be applied, viz.--a small portion of hydrochloric acid, and if there be effervescence it must be dissolved out by this means. From Algæ and other weeds diatoms may be detached by agitating the whole together in a weak solution of nitric acid--about one of pure acid to twenty or thirty of water, as it must be sufficiently weak to free the diatoms without destroying the matter to which they adhere. The diatoms may then be separated by sifting through coarse muslin, which will retain the Algæ, &c. The process of cleaning will vary according to circumstances. Some gatherings require to be boiled only a few minutes in nitric acid; but the more general plan where they are mixed with organic or other foreign matter, is to boil them in pure sulphuric acid until they cease to grow darker in colour (usually from a half to one minute), and then to add, drop by drop to avoid explosions, a cold saturated solution of chlorate of potash until the colour is discharged, or, in case the colour does not disappear, the quantity of the solution used is at least equal to that of the acid. This operation is best performed in a wide-mouthed ordinary beaker glass,[B] a test-tube being too narrow. The mixture whilst boiling should be poured into thirty times its bulk of cold water, and the whole allowed to subside. The fluid must then be carefully decanted and the vessel re-supplied once or twice with pure water, so as to get rid of all the acid.