H. MORLAND ON DIATOM STRUCTURE. 299 



thinness. I think I can safely affirm that some of my sections 

 are in parts not more than ^J^ of an inch thick, and even less ; 

 in fact, I should consider a section T ^ incn tnick bv no means 

 thin. But to obtain these very satisfactory results it is as well 

 to prepare a number of sections and then select the best. It can 

 be readily understood that when we have to deal with such exceed- 

 ingly thin sections the slightest amount of grinding at the finish, 

 one way or the other, will mean either a comparatively thick 

 section or none at all. 



To resume, on receiving my slices of " Cementstein " I smoothed 

 one side on a piece of thick glass with a little " Wellington knife 

 powder " and water. The powder, although fine, is too coarse to 

 finish off with, but being brittle it breaks up finer and finer as the 

 grinding proceeds, and eventually becomes quite fine enough for 

 our requirements, provided no fresh powder be added towards the 

 finish. I generally prepare some half-dozen slices at a time ; 

 when one has all the needful apparatus ready it is as well to make 

 full use of it. The slices being thin, say ^ of an inch, by pressing 

 the tip of one of the fingers on the centre of them they can be 

 finished off pretty level, as they spring and get ground down rather 

 more in the centre than they otherwise would, and thus is counter- 

 acted the rubbing away at the sides and corners which usually takes 

 place when sections are rubbed down by hand. When the slices 

 are duly finished off on one side, I then attach them with balsam, 

 prepared side downwards, to the slips on which they are finally 

 mounted. And here a few words of caution are necessary, viz., 

 " the balsam must be hard," for if it were not so the section, as it 

 approached the final grinding, would, in spite of every care, begin 

 to break up for want of a steady support, in consequence of the 

 balsam being soft and yielding. It is also necessary to avoid 

 bubbles under the section, as bubbles mean breaking away of the 

 section in those parts for want of proper support, but better, by 

 far, a bubble than soft balsam. I fix the slices with balsam 

 slightly hardened, and then harden off gradually by putting 

 the slips in a very cool oven for a week or ten days ; by so doing 

 I get the balsam hard throughout, and without bubbles. The 

 second side of the slice of " Cementstein ,: can now be rubbed 

 down similarly to the first side, with " Wellington knife powder" 

 and water on glass. As the section approaches completion, care 

 and very light pressure must be employed, and the " Coddington ' : 



