66 PREPARATION OF DIATOMS. 



diatoms are generally very silicious and are uninjured by being treated 

 with acid. This, however, is not the case with many marine diatoms, 

 which are very frequently only feebly silicious, and can only be sub- 

 mitted, therefore, to a moderate calcination at the utmost. 



In treating diatoms with acid a small quantity is placed in a test 

 tube, and covered over with one or two drops of nitric acid and the 

 whole boiled for a (ew seconds (or sometimes for one or two minutes) 

 in the flame of a spirit lamp, taking care that the dangerous vapours 

 emitted do not injure the lungs or any instruments of precision. This 

 operation should therefore be carried out in the open air or in the fume 

 chamber of a laboratory. 



When the tube has cooled, it is filled with distilled water and left 

 to rest. Some time afterwards the diatoms will be found collected at 

 the bottom of the tube, the supernatant liquid is then carefully decanted or 

 drained off and replaced by a fresh quantity of distilled water, and so 

 on until the water shows no trace of acid. Then the water is decanted 

 for the last time and the diatoms are finally covered over with a small 

 quantity of liquid ammonia, which, after the lapse of some hours is in 

 its turn decanted, and any traces of it that may then remain are got rid of by 

 successive washings with distilled water. After this last operation the diatoms 

 are perfectly cleaned and they are then placed in alcohol as previously stated. 



The mud of marine estuaries may be treated in the same manner, but 

 great care must be taken to use only distilled water for washing. It 

 has, in fact, the property of holding the clay in suspension, which can 

 then be easily removed by decanting. If calcareous or saline water be 

 used for the washings, the mud is precipitated with the diatoms. 



However, it sometimes happens that the diatoms are mixed with so 

 much organic matter (as is the case with guanos amongst others) that the 

 treatment above-mentioned is not sufficient. In such a case the mass 

 must be submitted to a more complicated treatment. This is carried 

 out in the following manner : 



The material is treated with nitric acid as above described, especially 

 when, as frequently occurs, the presence of calcium carbonate is suspected. 

 It is then carefully washed and dried. Witnout this preliminary opera- 

 tion the final product will contain a large quantity of calcium sulphate 

 crystals, which it would be almost impossible to get rid of. 



The material, having then been treated as above, is placed in 

 a deep porcelain evaporating dish ; a small quantity of concentrated 

 sulphuric acid is poured over it so as entirely to cover the diatoms, 

 and the whole is boiled for two or three minutes, 



