624 MICROSCOPIC FOKMS OF VEGETABLE LIFE THALLOPHYTES 



increasing intervals, until no further sediment 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 exclusively of diatoms, the sizes of 

 which will be so graduated that the earliest sediments may be 

 examined with the lower powers, the next with medium powers, 

 while the latest will require the highest powers a separation which 

 is attended with great convenience. 1 It sometimes happens that 

 fossilised diatoms are so strongly united to each other by siliceous 

 cement as not to be separable by ordinary methods ; in this case, 

 small lumps of the deposit should be boiled for a short time in a 

 weak alkaline solution, which will act upon this cement more readily 

 than on the siliceous frustules ; and as soon as the lump is softened, 

 so as to crumble to mud, this must be immediately washed in a large 

 quantity of water, and then treated in the usual way. If a very 

 weak alkaline solution does not answer the purpose, a stronger one 

 may then be tried. This method, devised by Professor Bailey, has 

 been practised by him with much success in various cases. 2 



The mode of mounting specimens of Diatomacece will depend 

 upon the purpose which they are intended to serve. If they can be 

 obtained quite fresh, and if it be desired that they should exhibit, as 

 closely as possible, the appearance presented by the living plants, 

 they should be put up in aqueous media within cement-cells ; but if 

 they are not thus mounted within a short time after they have been 

 gathered, about a tenth part of alcohol should be added to the water. 

 If it be desired to exhibit the stipitate forms in their natural position 

 adherent to other aquatic plants, the entire mass may be mounted 

 in Deane's medium or in glycerin jelly, in a deeper cell ; and such a 

 preparation is a very beautiful object for the background illumina- 

 tion. 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 being washed, should be boiled in 

 water with a small piece of soap, whereby the diatoms will be cleansed 

 from the flocculent matter which they often obstinately retain. 3 

 After a further washing in pure water, they are to be either mounted 

 in balsam in the ordinary manner, or be set up * dry ' on a very thin 

 slide. In order to obtain a satisfactory view of their markings, 

 objectives of very large aperture are required, and all the improve- 



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

 by Mr. Okeden in the Quart. Journ. Microsc. Science, vol. iii. 1855, p. 158. 

 The Author believes, however, that the method above described will answer every 

 purpose. 



2 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. A little book entitled Practical Directions 

 for Collecting, Preserving, Transporting, Preparing, and Mounting Diatoms (New 

 York, 1877), containing papers by Professors A. Mead Edwards, Christopher Johnson, 

 and Hamilton L. Smith, will be found to contain much useful information. 



5 See Prof. H. L. Smith in Amer. Journ. of Microscopy, vol. v. 1880, p. 257. 

 It is important that the soap should be free from kaolin, silex, or any other insoluble 

 matter. 



