230 Hoiv to prepare Specimens of Diatomacese for 



■well-worn cambric handkercliief. The covers, always round, 

 sliould be separated into sizes and thicknesses, so that the exact 

 kind of cover required can be found without having to search for it 

 by turning over a number, scratching or breaking them, and losing 

 much valuable time. We shall also require a pair of forceps for 

 holding the slides over the lamp ; and such as are sold at house- 

 furnishing stores and by grocers, under the name of American 

 clothes-pegs, and which have been already mentioned, are by far 

 the best I have ever seen or heard of. A small pair of brass forceps 

 which close with a spring will be needed, and they are best set in a 

 wooden handle so as to protect the fingers from the heat; and 

 another pair, which spring open and may be closed by means of 

 the finger and thumb, will be wanted for taking hold of and adjust- 

 ing the thin covers. I do not advocate the use of paper covers for 

 slides, but labels of some kind will, of course, be required, and I 

 have found the plain circular white ones to look the best. There 

 are very jDretty square labels sold by dealers in these things that I 

 have used and liked. For making cells to hold specimens put up in 

 a fluid, a turn-table and brushes and some cement will be necessary. 

 The cement I use and prefer above all others is good old gold size 

 used warm. 



The chemicals required are nitric acid, sulphuric acid, hydro- 

 chloric acid, bichromate of potash, caustic potash, alcohol, and, 

 above all, a plentiful supply of clean, filtered water. The water 

 should be such as leaves hardly any residuum when a quart 

 of it is evaporated to dryness ; and it must be filtered just before 

 use, to remove any minute organisms, diatoms especially, which it 

 may contain. A certain amount of washing soda will be wanted, 

 if guanos are to be cleaned. 



We will now proceed to consider the manipulations necessary 

 to prepare the various kinds of gatherings, always remembering 

 that these methods will have to be modified to a certain extent for 

 each specimen. 



Recent Gatherings. — If there be sand in the gathering, it will 

 be well to remove it before using acid, by shaking it in clean water 

 and pouring ofi" before the diatoms, which are lighter than the 

 sand, settle. The water holding the diatoms in suspension may be 

 poured into a test-tube, or beaker, the diatoms allowed to settle, 

 and as much of the water poured ofi" as possible. The diatoms are 

 now covered with nitric acid to about the height of half an inch, 

 and allowed to stand for a few minutes. Usually some chemical 

 action takes place, and it will be well to wait until it subsides. The 

 test-tube or beaker is then held over the lamp and carefully heated 

 until the reaction of the acid upon the organic matter of the 

 diatoms ceases. Thereafter, and while the liquid is still hot, I have 

 found it often advantageous to drop in one or two fragments of 



