Examination and Study hy means of the Microscope. 231 



bicliromate 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, washing 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 coloured chromium 

 compound has been removed. When this point is arrived at can 

 only be ascertained from experience. In this way the valves and con- 

 necting membranes of the diatoms are usually separated and cleaned 

 ready for mounting, which process will be described hereafter. 



Muds will have to be treated in a somewhat different manner 

 from recent gatherings. If the mud is dry, it will have to be 

 broken down by boihng for a few minutes in a solution of caustic 

 potassa, the strength of which must be apportioned to the particular 

 specimen under treatment. After it has been broken down into a 

 soft mud, all of the potash is thoroughly washed off by means of 

 clean water, and replaced by nitric acid, as in the case of recent 

 gatherings. This is boiled, and a little bichromate of potash added 

 as before, and the whole washed. It very seldom happens that the 

 diatoms occurring in mud will be sufficiently cleaned by this process, 

 so that it has to be supplemented by another. The sediment is 

 therefore washed into one of the evaporating dishes and allowed to 

 settle, and as much of the water poured off as possible. Then 

 sulphuric acid, in quantity to a little more than cover them, is 

 poured in, and the vessel gradually and carefully heated. As soon 

 as the liquid shows signs of boiling, bichromate of potash is added, 

 a very little at a time, until the green colour first formed by its 

 reaction upon the organic matter begins to assume a yellowish tint, 

 when no more is dropped in ; but a few drops of hydrochloric acid 

 are permitted to fall in, and the liquid is allowed to cool. Of course 

 it will be best if the person undertaking to clean diatoms is some- 

 what versed in the use of chemicals ; but at any rate care must be 

 taken not to drop any of the acids upon the clothes or skin, and 

 great caution must be exercised in not inhaling any of the vapours 

 given off. Those evolved after the addition of the hydrochloric acid 

 are especially irritating and dangerous, and must be avoided. As 

 soon as the liquid has cooled a little, water should be added 

 cautiously, as great heat will be generated thereby, and there will 

 be danger of its boiling over. Thereafter it may be poured into a 

 large beaker-glass of water and thoroughly washed as in the former 

 case. If it be found that the precipitate is not quite white, it will 

 be necessary to boil it again in sulphuric acid, with bichromate of 

 potash and hydrochloric acid, until it is quite clean. If, on examina- 

 tion by means of the microscope, it is found that there is much 



