492 PHYSICAL GEOGRAPHY. 



in these things that I have used and liked. For making cells to hold 

 specimens put up in 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, hydrochloric 

 acid, bichromate of potash, caustic potash, alcohol, and, above all, a plen- 

 tiful 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, dia- 

 toms 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 pre- 

 pare 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 

 off 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 off 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 mat- 

 ter 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 

 bichromate of potash. The organic matter is more thoroughly destroyed 

 in this way than when the acid is used alone. Thereafter it is well to 

 pour the acid and diatoms into a capacious beaker of clean water, wash- 

 ing the tube or smaller beaker out with a little water, and adding this to 

 the other. After the diatoms have all settled, which will often require 

 hours, the supernatant fluid is carefully poured off, and a fresh supply 

 added ; and this must be repeated several times until all of the acid and 

 colored chromium compound has been removed. When this point is 

 arrived at can only be ascertained from experience. In this way the 

 valves and connecting membranes of the diatoms are usually separated 



