Page:Hardwicke's Science-Gossip - Volume 1.pdf/164

The excellent article on "Cleaning Diatomaceæ" in the number of Science Gossip for March leaves but little to be desired on that point. Yet, notwithstanding all the care that may be taken, many operators, especially those on guano, will fell dissatisfied with the results they obtain, on account of the mass of foreign matters which, with all their care, will still cling to their deposits, and mar the beauty of their slides. To those who may be desirous to obtain fair preparations without this defect, I would recommend a modification of a plan hinted at by Carpenter, and which I have practised with some success.

I take a slide, well cleaned, and with a dipping-tube put a narrow line of the deposit along its margin: the line should not be more than one inch long, and a quarter of an inch broad. I then place the slide under a simple microscope of moderate power, and I can easily distinguish the different diatoms, event those that are small. I select a large one at first, and with a stout needle I push it out of the water, a thin stream following towards the centre of the slide. When near the centre, I raise up gently the remote edge of the slide, and then push the diatom out a little, when it becomes "high and dry" on the centre of the slide. In a similar manner, one after another, I put out as many as I please to select, the process becoming easier as I proceed, as the water returns more readily with the needle, and leaves the diatom dry in its proper place. Having exhausted the store of select diatoms in the line, I wipe it off carefully without touching the dry diatoms, and then I put down with the dipping-tube another similar narrow line, and proceed as before. I have, in this way, on one slide, taken the choice diatoms out of twenty such narrow lines. No doubt it required time and skilful manipulation, but it became very pleasant and interesting, every fresh portion of deposit laid down affording fresh objects for search, and sometimes new prizes for preservation. The number of diatoms which I place on one slide varies according to the circumstances. On one occasion the first search gave me a specimen of Auliscus formosus: this I put out and mounted alone, a perfect disc, and a beautiful object, which of itself repays me for the time occupied. At another time, after putting out a number, I obtained a valve with eight highly inflated marginal processes, an example of which I have not yet seen figured. I was then satisfied, and mounted the slide; but in general I proceed until I have nearly two hundred perfect and clean discs nicely placed in the centre of a slide; and in no case have I occupied more than one long evening of a summer day in the preparation and mounting of one of my slides.

When a sufficient number of diatoms have been obtained, they appear to the naked eye as a small circular white spot on the slide. I then heat the slide on a brass table, and take some balsam on a stout wire, and place a thin circle of the balsam round the diatoms, taking care to place just so much as may be required. I then drop a circular cover on the balsam, which speedily runs in and fills the space under the cover without disturbing the diatoms. I then place the slide in a drawer, lying flat for a week or two, until the balsam becomes tolerably hard.

The advantages of the above method will be, in part, apparent from what I have said; I may, therefore, only mention, that by it the student would become quite familiar with the appearances of diatoms in water, dry, and in balsam; and it is needless to say that these appearances vary, and are instructive. Further, slides can thus be obtained, each containing only one sort of diatom, and this will be often desirable for scientific purposes.

Before closing this paper, I wish to mention a few of the results of my observation in working Peruvian guano by the method I have given. Auliscus Peruvianus, which has been figured in the Microscopical Journal, is a diatom rather small; in water it appears of an even tawny or yellow colour, delicate, and not very attractive in appearance. When dry it becomes quite opaque, and the two processes of a single valve shine brightly in the gloom of the general appearance.

In balsam it becomes very transparent, and requires a tolerably good glass and proper illumination to display the faint markings of the now interesting valve. Another diatom of the class Auliscus appears in water reddish and delicate—dry it seems dark and lurid—and in balsam grey, with sometimes a reddish centre when seen under a low power. Nothing could be more different than the appearances of these diatoms in the three different mediums.

Again, a small diatoms of the class Asterolampra, when put out of water with a needle, dries quickly, becomes quite dark in colour, and then jumps like flea, an inch or two to one side, or it may be off the slide altogether. Should it be on the slide, it can easily be pushed back dry to the others; but if the needle be so damp as to wet the diatom, it quickly dries, and then jumps again. Diatoms of this class are, for the most part, most distinctly marked when dry.

Many other things worth note might be added, but enough has been said for the present. In giving the above, I do not mean to advocate the exclusive adoption of my plan. It is well that various methods should be in use, as each may lead to the discovery of facts contributing to the mass of general knowledge. Besides, many may be deficient in patience and manipulative skill for success in its adoption; but I feel persuaded that many more who fail in cleaning their gatherings and mounting