TECHNOLOGY OF THE DIATOMACE^. 31 



in a slow oven, taking great care to avoid all contact with any 

 greasy matters." 



When perfectly dry, the mud is broken into fragments, if it has 

 agglomerated in drying, and thrown into a vessel sufficiently large 

 and filled two-thirds of water (a tumbler-glass or a celery glass, 

 for instance). The whole is rapidly mixed, and then after one or 

 two minutes of repose carefully remove, by means of a spoon, a 

 kind of scum that will be seen to float on the surface. It is spe- 

 cially on the edges and against the sides of the vessel that the 

 scum is most abundant, and one may use with advantage a large 

 soft brush to remove it, pressing lightly all round the vessel. This 

 operation, if the mass is considerable, may be repeated twice or 

 even three times. 



If the mud contains Diatoms, an examination under the 

 microscope will show a great abundance of frustules, filled with 

 air, which has caused them to float on the surface of the water. 

 If nothing is seen after a treatment of this kind, it is a proof that 

 the sounding is sterile, and may then be rejected. (For fossil 

 earths of the same nature the case is different, and the absence of 

 Diatoms in the floatage is not to be taken as a proof of sterility.) 



The collected scum, placed in a porcelain capsule with a little 

 water, is brought to the boiling point for a few minutes to drive 

 the air out of the Diatoms, which on cooling will collect at the 

 bottom of the vessel and may then be treated with acids. 



Extraction of the Diatoms remaining in the aforesaid floated 

 materials.— Many valves and frustules of Diatoms may be 

 extracted from the mud already floated by the following means : — 



I St. — Tamise the whole with ordinary muslin of sufficient 

 solidity, and having meshes of a millimetre, which will allow all 

 the Diatoms to pass through, retaining only the more bulky 

 rubbish — shells, stones, etc. — which may be thrown away. 



2nd. — The deposit thus obtained may be again tamised, 

 employing the No. i or 2 filter, according to the size of the 

 Diatoms contained. When the water passes through clear, you 

 must treat the part that remains on the filter with a series of acids 

 and re-agents till it is perfectly cleaned. 



* Mr. W. H. Shruhole has made known this process, which he has 

 employed, for removing the Diatoms contained in the mud of the Thames, in 

 The Leisure Hour, 1 891, p. 387. 



