OF MICROSCOPIC OBJECTS. 67 



As an occasional aid, it may be remarked, that in some 

 oases liquor ammonice may be used in one of the later 

 washings in place of water, as it often separates fine dirt, 

 which is not otherwise easily removed. Ammonia also dis- 

 solves a flocculent matter which sometimes remains; and 

 this method does not injure diatoms like some strong alkalies. 

 Some fossil deposits require to be treated with a boiling 

 solution of carbonate of soda to disintegrate them; but 

 this operation requires great care, lest the alkali should 

 destroy the diatoms. Vegetable silicates also sometimes 

 require to be removed by a solution of carbonate of soda ; 

 but as the frustules of the diatoms themselves are but 

 vegetable silica, even more care is required in this case. It 

 may be well to mention, that some diatoms are so imperfectly 

 siliceous that they will not bear boiling in acid at all. Some 

 f)f these may be allowed to stand in cold nitric acid some 

 time, whilst others of a smaller and more delicate character 

 should, when possible, be treated with distilled water 

 alone. 



We will now consider the mode of mounting the prepared 

 diatoms, which, if used dry (as described in this chapter), 

 should be carefully washed two or three times with the 

 purest distilled water. In this branch, as in every other, 

 each collector gives preference to that method in which he 

 is an adept. Thus the diatoms may be placed on the 

 under side of the cover, to be as near to the object-glass 

 as possible, or upon the slide itself; and each plan has 

 its advocates. Whichsoever of these is used, nothing seems 

 more simple to the novice than a tolerably equal dispersion 

 of the objects upon the slide or cover; but this is by no 

 means so readily accomplished ; consequently I give Mr. 

 Rylands' method, as his elides are perfect in this respect 

 also. He always places the diatoms upon the thin glass 

 cover. It is not sufficient, as is frequently thought, to 

 take a drop of liquid containing the cleansed material and 

 spread it upon the cover or slide, as without some additional 

 precaution that uniform and regular distribution of the 

 F 2 



