316 MODES OF MOUNTING DIATOMACEJE. 



then be tried. This method, devised by Prof. Bailey, has been 

 practised by him with much success in various cases.* 



238. The mode of mounting specimens of Diatomacese will de- 

 pend 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 (§ 166) ; 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 Deane's Medium (§ 163) 

 or in Glycerine Jelly 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 (§ 160), or, if the species have 

 markings that are peculiarly difficult of resolution, is to be set up 

 dry between two pieces of Thin-glass (§ 154). In order to obtain 

 a satisfactory view of these markings, Objectives of very wide 

 Angular aj)erture are required, and all the refinements which have 

 recently been introduced into the Methods of Illumination need to 

 be put in practice. (Chaps, in., 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 removed by the usual means. The larger 

 forms, which may be readily distinguished under a simple Micro- 

 scope, 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. 

 (Such split hairs may always be found in a Shaving-brush which 

 has been for some time in use ; those should be selected which 

 have their split portions so closely in contact, that they appear 

 single until touched at their ends.) When the split extremity of 

 such a hair touches the glass slide, its parts separate from each 

 other to an amount proportionate to the pressure ; and, on being 

 brought up to the object, first pushed to the edge of the fluid on 

 the slide, may generally be made to seize it. — Supposing that we 



* For other methods of cleaning and preparing Diatoms, see " Quart. 

 Journ. of Microsc. Science," Vol. vii. (1859), p. 167, and Vol. i. N.S. 

 (1861), p. 143 ; and "Trans, of Microsc. Soc," Vol. xi. N.S. (1863), p. 4. 



