NATURAL HISTORY OF THE DIATOMACE^E. 493 



and cleaned ready for mounting, which process will be described here- 

 after. 



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 

 boiling 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 thor- 

 oughly 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 hap- 

 pens that the diatoms occurring in mud will be sufficiently cleaned by 

 this process, so that it has to be supplemented by another. The sedi- 

 ment is therefore washed into one of the evaporating-dishes and allowed 

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

 phuric 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 color 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 under- 

 taking to clean diatoms is somewhat 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 vapors given off. Those evolved after the addition of the hydro- 

 chloric acid are especially irritating and dangerous, and must be avoided. 

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

 tiously, 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 examination by means of the micro- 

 scope, it is found that there is much flocculent matter present besides 

 the diatoms and sand, this can be removed by boiling for a few seconds 

 in a weak solution of caustic potash, and washing quickly and thoroughly 



