ON MOUNTING. 147 



the case, there has been animal matter present, 

 which has not been entirely removed, boil in pure 

 sulphuric acid for a few minutes, and wash away 

 all trace of the acid before placing the diatoms in 

 distilled water, for keeping. 



To clean Fossil Diatomaceous Deposits.- 

 Sorne fossil deposits are of such extreme hardness 

 that it is necessary to resort to caustic potash for 

 their disintegration; it is, however, better, when- 

 soever possible, to dispense with this powerful 

 alkali, since, even in the most careful and practised 

 hands, more or less abrasion of the surface of the 

 valves will generally be found to result from its 

 use. When a fossil deposit will not yield to milder 

 treatment, it must be broken into small pieces, 

 placed in a test-tube, just covered with the solution 

 of caustic-potash, and boiled for half a minute 

 (only) over the flame of a " Bunsen " burner ; 

 pour off into a glass beaker nearly full of water 

 all the deposit which has been disintegrated by 

 the potash and repeat the process until all the 

 lumps are broken up. Then, when all the forms 

 have settled to the bottom of the water in the 

 beaker, pour off the water, remove the diatoms into 

 a large test-tube, fill the test-tube nearly full ot 

 water, add a small quantity of bicarbonate of soda, 

 and let the diatoms gently "simmer" in this weak 

 solution for two hours. Wash away all soda, and 

 boil the diatoms for a few minutes in pure nitric 

 acid. Remove all trace of acid by repeated wash- 

 ings, and keep the diatoms in distilled water. 



Fossil deposits which can be disintegrated without 

 caustic potash should be boiled in a test-tube of 

 large capacity, in a moderately strong solution of 



