1894.J MICROSCOPICAL JOURNAL. 83 



dry, and afterward shrinks greatly if allowed to dry com- 

 pletely. This shrinkage crushes the diatoms into frag- 

 ments, particularly if such large and fragile species as 

 Pleurosigma halticum and P. terry unum are concerned. 

 In soundings and in fossil deposits, the large amount of 

 tine sand protects the diatoms to some extent, so that 

 they are not so certain to suffer in drying, but fossil ma- 

 terial should be handled with great care. I frequently 

 see it sent out pulverized, as if the senders imagined that 

 such small forms could not be broken. Some recommend 

 breaking down refractory material by soaking in water 

 then freezing and thawing, this is ingenious but slow ; 

 why not use a hammer ? It would be quicker and not 

 much more destructive. I have sometimes found that 

 in digging fossil earth the pressure of the spade in lift- 

 ing out blocks of material would compress the earth suf- 

 ficiently to crush millions of the best diatoms it con- 

 tained. 



On Cements for the Microscope. 



By ARTHUR M. EDWARDS, M. D., 



NEWARK N. .1. 



With the danger of trespassing on the ground occupied 

 by other observers and more thoroughly worked over by 

 them, I nevertheless venture to write what I have arrived 

 at in the way of using cements for the preparation of mi- 

 croscopic objects. I have used them for over forty years 

 and have always turned back to English gold -size, as be- 

 ing the best to use in all cases. It is tough and dries 

 readily enough, being about a day in drying. 



Now what I use is this : Some modification of Shad- 

 boldt's turn-table is necessary. My friend CoLTCinne of 

 San Francisco, made the first and on all accounts best, I 

 believe. But to Mr. George Wale is due the iron stand. 

 I remember, over twenty years ago, when he brought it 







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