Before mounting, Mr. Rylands always burns the diatoms upon the glass at a dull red heat, whether they are used with balsam or dry. This burning, he says, is not only an additional cleaning process, but it effectually fixes the diatoms, and prevents them floating out if mounted with balsam. The thinnest covers may be burnt without damage if they are placed upon a small piece of platinum foil of the size required, which should be about one-hundredth of an inch thick, perfectly flat, and having three of its edges slightly bent over, so as to prevent its warping with the heat. The small flame of a spirit-lamp, or, where there is gas, a Bunsen’s burner, may be employed. The cover should be shaded from direct daylight, that the action of the flame may be observed more perfectly. Care must then be taken to raise the temperature only to the dull red heat before mentioned. The cover will then be in a fit state for mounting as required.
It has been stated in another place that it is assumed the operator is not mounting diatoms simply as microscopic objects, but as instructive specimens. It is not, therefore, sufficient to take a single slide as all that is required, but to have the same diatom prepared in as many ways as possible. The following are the principal:—
1. Mounted crude in fluid (see [Chapter IV.]).
2. Burnt crude upon the cover, and mounted dry or in balsam (as before mentioned).
3. Mounted dry or in balsam (see [Chapter III.]), after the cleansing process already described.
I will here give Mr. Rylands’ method of mounting them dry, the fluid and balsam preparations being noticed in their respective chapters. The slide with the ring of asphalt, or black varnish, should have been prepared some weeks previously, in order to allow it to dry thoroughly. When required, it must be held over the spirit-lamp or Bunsen’s burner until the ring of varnish is softened. The burnt cover, having been heated at the same time, must then be taken in the forceps and pressed upon the softened varnish until it adheres all round. When cold, an outer ring of asphalt completes the slide.
Such is the method which my friend Mr. T. G. Rylands employs in the preparation of diatoms for the microscope. I have said enough concerning his results. It is to be feared, however, that to some these several modes of operation may appear lengthy and complicated; but if read carefully, and the experiments tried, they will be found simple enough in practice, and to occupy much less time than an intelligible description would lead the novice to believe necessary.
One of the most fertile as well as the most curious magazines of Diatomaceæ is guano. The siliceous forms contained therein have been devoured by sea-birds and passed through the stomach uninjured, and after lying for ages may be cleaned and classified. Many of these are not elsewhere met with, so that the student who is desirous to enter into the study of Diatomaceæ must be instructed as to the best mode of obtaining them from this source. The particulars to be observed so closely resemble those before mentioned in the treatment of the ordinary diatoms, that it will be sufficiently explicit to give the outlines of the process. The guano must be first washed in pure water, allowed to subside perfectly, and the liquid then poured off. This must be repeated until the top fluid is clear, and care taken not to decant the liquid until perfect subsidence has taken place. The deposit must then be treated with hydrochloric acid with a gentle heat for an hour or two, adding a little fresh acid at intervals as long as it excites any effervescence After this nitric acid must be substituted for the hydrochloric, and the heat kept up to almost boiling-point for another hour at least, adding a little fresh acid as before. When this ceases to act, the deposit must be allowed to settle perfectly and the acid poured off. All traces of the acid must now be washed away with pure water, when the remains will be Diatomaceæ, the sand contained in the guano, and a few other forms. Some of these may be mounted dry, as before mentioned, but the greater portion should be put up in Canada balsam as described in [Chapter III.]
Such is the ordinary method for the treatment of guano; but Mr. Rylands’ mode of proceeding with ordinary Diatomaceæ (before given) will be found equally successful with these deposits.
The fossil Infusoria (as they were formerly called) are now termed Diatomaceæ, and are found in various parts of the world—“Bermuda earth,” “Berg-mehl” from Norway, deposit from Mourne Mountain in Ireland, &c. They are found in immense quantities, and afford the microscopist innumerable objects. The same treatment as that usually employed for the Diatomaceæ must be followed with these deposits, but as they are sometimes obtained in hard masses, disintegration is first necessary. To effect this, they are usually boiled for a short time in diluted liquor potassæ, which will soon cause the mass to fall into a mud-like deposit. Water must then be immediately added, in order that all further action of the liquor potassæ may be stopped, otherwise the objects searched for will be dissolved. For this reason it is necessary to understand what substance is being dealt with, because some deposits are much finer and acted upon more readily than others.
In mounting these objects, some are so delicate that they are almost invisible when balsam is used with them; they are therefore usually mounted dry. Others, however, are much coarser, and may be mounted in balsam like the Diatomaceæ mentioned in [Chapter III.]
The common Infusoria cannot be mounted dry with any great success, though a few may be placed upon the glass slide and allowed to dry naturally, when their characters will be very well shown. To obtain anything like a natural appearance, they must be put up in fluid as in [Chapter IV.]