OF THE DIATOMACE^. 231 



now be set aside to cool, when the deposit must be finally washed 

 as has been already described. There is another process which is 

 substantially the same as the one already described, but with this 

 exception. Sulphuric Acid is substituted for Nitric Acid, and 

 crystals of Nitrate of Potash for those of bichromate. This has, 

 I think, some advantages over the first-mentioned process, and, as 

 I invariably follow it^ I have the greatest confidence in recom- 

 mending it. When a recent gathering is nearly pure, all that is 

 required is to burn it on a platinum plate over a gas flame, the 

 organic matter being quickly carbonised, while the beautiful 

 siliceous shields alone remain unafl'ected. 



Guanos. — The preparation of these substances so as to obtain 

 the Diatoms mixed with them, is unquestionably tedious and dirty; 

 but I would qualify this somewhat discouraging remark by stating 

 that these ammoniacal guanos contain by far the most beautiful 

 forms, and the result is therefore well worth the labour bestowed. 

 As a type of this we will take some Peruvian guano, first sifting it 

 to free it from stones, feathers, and other debris. The finely-sifted 

 material should be slowly dried in an oven, which causes the 

 evaporation of a considerable portion of the ammonia, and most of 

 the moisture with which it is so frequently charged. A tin pan or 

 skillet is now to be half-filled with a strong solution of commercial 

 Carbonate of Soda, about three ounces of sodcE carb. to the pint of 

 water, and placed over a gas-stove, and on the liquid boiling the 

 guano is gradually and slowly dropped in. It is necessary 

 frequently to stir the solution to prevent its boiling over, owing to 

 a considerable effervescence produced by the Soda on the 

 Ammonia of the guano. When the liquid ceases to effervesce, it 

 is poured into a plentiful supply of clean water, and washed 

 several times, taking, of course, every care that the frustules of the 

 Diatoms are not washed away in the process. A red-looking mud 

 is the result of this process, which, on boiling in Sulphuric Acid, 

 treating with Bichromate of Potash, and finally washing, will yield 

 some very clean and beautiful specimens. The best guanos for 

 treatment are without question those of Pabellon de Pica, Isle of 

 Maccabees, Patos, and Old Ichaboe. In reviewing the work done, 

 it will be well to consider the effect of the acids used, always 

 remembering that Hydrochloric has a low boiling point, Nitric 



