72 PREPARATION OF DIATOMS. 



fall to the bottom ; then I decant the liquid which will contain fine 

 sand and small diatoms. To separate the sand from the latter I have 

 successfully pursued the following method : 



" I take two glass slips A and B. On A I place a drop of distilled 

 water and on B a small quantity of the liquid to be treated. I give 

 B a slight rotatory movement, keeping it in a horizontal position, which 

 accumulates the sand in the centre of the liquid, and I then incline 

 the slip towards one of its corners ; the diatoms are drained off and 

 fall into the drop of distilled water on slip A. This water is then 

 spread on cover-glasses. By this method some gatherings which seem 

 to be quite worthless furnish very good preparations, although it is 

 often necessary to add several drops of B to the slip A. I recom- 

 mended this method of working to one of my friends who complained 

 of the trouble which a gathering of the mud from a rice plantation 

 was causing him. In thanking me shortly afterwards he expressed his 

 astonishment at the number of different forms he had obtained from 

 this gathering, which at first appeared to be valueless. 



"The preparation of deposits of marine origin is generally very diffi- 

 cult in consequence of the diatoms being apparently united together by 

 a silicious cement which it is very difficult to get rid of without 

 destroying the diatoms. I succeeded completely however by proceeding 

 as follows : -A small portion of the material to be treated having been 

 placed in a test tube I add some nitric acid, I boil it and wash to 

 get rid of all lime. I then treat it with sulphuric acid and decolourise 

 the mass which has generally turned brown under the action of the 

 potassium chlorate. After a careful washing, the mass is boiled again 

 for a minute in a small quantity of water, to which a little sodium 

 carbonate has been added. After a subsequent washing the mass is 

 returned to the test tube and shaken until it disintegrates. If this 

 result is not obtained it should be boiled again in a solution of caustic 

 potass, and just as it falls to pieces it should be poured into water 

 to which some hydrochloric acid has previously been added. 



" When a gathering of living diatoms has to be dealt with, and 

 especially forms which develop in chains, a portion of the gathering 

 is treated with acid which is added to the other portion which has 

 not been used. In this way preparations are obtained in which the 

 two faces of the frustule and its mode of development can be studied 

 at the same time. 



" I have often tried to use the method recommended by Professor 

 H. L. Smith, but I have never been so successful as when I have 



