300 H. MORLAND ON "DIATOM STRUCTURE. 



used every minute or so. I generally grind till, in spite of all my 

 care, the section begins to break away at the edges. I now wash 

 the slip with section attached in clean water, wipe same and dry 

 off with a very gentle heat, not sufficient to soften the balsam. 

 Having got a cover glass ready, I place a very small quantity of 

 thickish balsam on the section, put on the cover, and press down 

 hard. If the right quantity of balsam has been added, there is 

 only sufficient to fill up under the cover, and, if done at once, the 

 cover can be adjusted to one side or the other, as may be found 

 necessary ; but this cannot be done later on, as by that time the 

 small amount of softer balsam under the cover will have attacked 

 and slightly softened the hard balsam under the section, and 

 moving the cover to one side or the other will have a tendency to 

 break up the section. I now place the slide in a cool oven for a 

 few days, and when labelled it is finished, unless I add a ring of 

 Bell's cement, which will enable the section to be examined under 

 an oil immersion lens without fear of the oil attacking and soften- 

 ing the balsam. 



Thus far I have only described how very thin sections of 

 " Cementstein " can be prepared ; I will now proceed to describe 

 how the diatom sections can be separated and isolated. After 

 preparing one side of the slice of " Cementstein," I attach it to a 

 piece of glass some lj"xl", instead of to an ordinary 3" xl" 

 slips and rub it down to the necessary thinness. I then immerse 

 it, still attached to the glass, in benzole. After a little time, say 

 half-an-hour, it can be brushed off with a camel-hair pencil on to 

 a glass slip and cleaned of all balsam by being brushed with the 

 camel-hair pencil charged with benzole. I then transfer the slice 

 to some methylated spirit to get rid of the residue of benzole, and 

 after a short time to a little clean water in a watch-glass. I now 

 pour off the water and add a few drops of hydrochloric acid, which 

 at once separates the diatoms contained in the section of 

 " Cementstein." I fill up the watch-glass with distilled or filtered 

 rain-water, allow to settle, draw off the liquid as close as I can 

 venture by means of a fine pipette, and fill up with water again ; 

 this I repeat several times, until I feel satisfied that I have got rid 

 of the hydrochloric acid. I next give the diatoms a boiling in 

 sulphuric acid in the watch-glass, which I place on a small piece 

 of wire gauze and apply the spirit lamp underneath. After washing 

 away the acid I have the clean diatom sections ready for selecting 



