232 COLLECTION AND PREPARATION 



Acid boils at a moderately high temperature, and Sulphuric Acid 

 the highest of all ; so that every attention must be paid to the 

 quality of the test tubes or flasks used. The primary use of 

 the acids is to remove all the lime compounds, and to assist in 

 the destruction of vegetable matter ; the addition of the Nitrate 

 of Potash is to bleach the residuum, but the sand remaining, 

 being indestructible, can only be got rid of by frequent washings 

 and decantations. 



Lacustrine and sedimentary deposits as those of Dolgelly, 

 Mull, Mourne Mountain, Kieselgiihr, Franzenbad, and Loch 

 Kennard are so pure that little more is required than boiUng in a 

 weak solution of Caustic Potash, and subsequent washing. Should 

 there be an unusual quantity of organic matter, recourse must be 

 had to Bergen's method, which is the Sulphuric Acid and Nitrate 

 of Potash treatment, already described. 



. Marine and fossil deposits of the character of South Naparima, 

 in the island of Trinadad, Moron in Spain, and the so-called 

 Marls of the island of Barbadoes, and all those of a stony and 

 rough nature, must, before boiling with Sulphuric Acid, go through 

 that operation in Liquor FofasscB, till the whole mass is broken 

 down into a soft mud. The liquid containing this in suspension 

 is poured into hot water, and after a space of three or four hours 

 all Diatoms will have fallen to the bottom of the vessel, the 

 resultant mass must be boiled in Hydrochloric Acid for about 

 twenty minutes, then a few drops of Nitric Acid added, again 

 washing out the acids and finally boiling with Sulphuric Acid, and 

 decolourising with Nitrate of Potash. The work of isolation and 

 separation into densities must now be proceeded with, and I 

 subjoin Professor Meade Edwards' method which I prefer to many 

 others, as I venture to think it is the easiest and shortest. Into a 

 beaker-glass of the capacity of an ounce, the cleansed Diatoms are 

 to be poured, filling up the glass with clean water ; this is then 

 stirred with a glass rod, and after an interval of six seconds,' 

 poured slowly into a larger vessel, taking care not to disturb the 

 sand or earthy matter which may have settled. The beaker must 

 be again filled with water, stirred, and allowed to settle for the 

 same interval, and poured into the same receptacle. When this 

 has been repeated about six times, all the sand free from Diatoms 



