230 COLLECTION AND PREPARATION 



be held over a spirit-lamp and carefully heated until the reaction 

 of the organic matter ceases ; and, while the liquid is still hot, 

 Professor Meade Edwards recommends the addition of one or two 

 fragments of Bichromate of Potash to bleach what organic matter 

 may still remain. 



The next step in the process is to pour the contents of the 

 test tube into a jar of clean water, at the same time rinsing out the 

 test tube, and adding the result to the rest of the water containing 

 the Diatoms, which should now be allowed to settle for three or 

 four hours at least. This being accomplished, pour off about two- 

 thirds of the fluid, adding fresh water, and repeating the process 

 till all acidity is removed, which may be proved by its ceasing 

 to redden litmus paper. Careful attention to this particular will 

 prevent the operator falling into a common error, whereby many 

 beautiful specimens are rendered unsightly when mounted. The 

 residuum will, in all probability, consist of a white flocculent 

 mass, which must be transferred to a bottle containing water and 

 a few drops of Carbolic Acid or a little alcohol, to prevent the 

 formation of confervoid growth, and the matting together of the 

 diatoms. 



Mud, according to Professor Edwards, requires to be treated 

 in a somewhat different manner. If it is dry and lumpy, it will 

 have to be broken down by boiling for a few minutes in a mixture 

 of Liquor Poiassce and water, in equal parts; after it has passed 

 into the state of soft mud, all the potash must be washed 

 out, by frequent additions and decantations of clean water. To 

 the mud thus prepared Nitric Acid must be added, as in the case 

 of recent gatherings, followed by the use of the crystals of 

 Bichromate of Potash, as already described. 



It not unfrequently happens that the Diatoms are insufliciently 

 cleaned by this means ; the sediment must, in such case, be 

 poured into an evaporating dish, with sufficient pure Sulphuric 

 Acid to cover it, and the vessel gradually and carefully heated. 

 To avoid any chance of explosion, so soon as the white fumes of 

 the acid appear, Bichromate of Potash should be added in small 

 portions, and when the green colour formed by its reaction 

 on the organic matter begins to assume a yellowish tint, a few 

 drops of Hydrochloric Acid must be added. The liquid should 



