806 MICROSCOPIC FORMS OF VEGETABLE LIFE. 



it should again be poured off; and this process may be repeated 

 three or four times at increasing intervals, until no further sedi- 

 ment subsides after the lapse of half an hour. The first sediment 

 will probably contain all the sandy particles, with, perhaps, some 

 of the largest Diatoms, which may be picked out from among 

 them ; and the subsequent sediments will consist almost exclu- 

 sively of Diatoms, the sizes of which will be so graduated, that 

 the earliest sediments may be examined with the lower powers, 

 the next with the medium powers, while the latest will require 

 the higher powers, a separation which is attended with great 

 convenience. 1 



193. The mode of mounting specimens of Diatomacese, will 

 depend upon the purpose which they are intended to serve. If 

 they can be obtained quite fresh, and it be desired that they 

 should exhibit, as closely as possible, the appearance presented 

 by the living plants, they should be put up in distilled water 

 within cement cells ( 134) ; but if they are not thus mounted 

 within a short time after they have been gathered, about a sixth 

 part of alcohol should be added to the water. If it be desired 

 to exhibit the stipitate forms in their natural parasitism upon 

 other aquatic plants, the entire mass may be mounted in Dearie's 

 gelatine ( 131), in a deeper cell; and such a preparation is a 

 very beautiful object for the black-ground illumination. If, on 

 the other hand, the minute structure of the siliceous envelopes 

 is the feature to be brought into view, the fresh Diatoms must 

 be boiled in nitric or hydrochloric acid, which must then be 

 poured off (sufficient time being allowed for the deposit of the 

 residue), and the sediment, after repeated washings, is to be 

 either mounted in balsam in the ordinary manner ( 128), or, if 

 the species have markings that are peculiarly difficult of resolu- 

 tion, is to be set up dry between two pieces of thin glass ( 122). 

 In order to obtain a satisfactory view of these markings, object- 

 glasses of very wide angle of aperture are required, and all the 

 refinements which have recently been introduced into the me- 

 thods of Illumination, need to be put in practice. (Chaps. Ill, 

 IV.) It will often be convenient to mount certain particular 

 forms of Diatomacese separately from the general aggregate ; but 

 on account of their minuteness, they cannot be selected and re- 

 moved by the usual means. The larger forms, which may be 

 readily distinguished under a simple microscope, may be taken 

 up by a camel-hair pencil, which has been so trimmed as to 

 leave two or three hairs projecting beyond the rest. But the 

 smaller can only be dealt with by a single fine bristle or stout 

 sable-hair, which may be inserted into the cleft end of a slender 

 wooden handle; and if the bristle or hair should be split at its 

 extremity in a brush-like manner, it will be particularly useful. 



1 A somewhat more complicated method of applying the same principle, is described 

 by Mr. Okeden in the " Quart. Journ. of Microsc. Science," vol. iii, p. 158. The Author 

 believes, however, that the method above described will answer every purpose. 



