12 THE AMERICAN MONTHLY [Jan 



a ring of another cement, having a purely alcoholic or 

 maphtha solvent. 



Such a cement is Ward's brown cement. In some re- 

 spects this cement resembles Miller's caoutchouc cement, 

 but it contains no caoutchouc and is more finely grained 

 and better in quality ; it dries with a fine gloss, never 

 cracks, and is not attacked by benzole or turpentine ; its 

 composition is not known, and its best solvent is a mix- 

 ture of wood naphtha and methylated spirit. 



Taking all these considerations into account, I have of 

 late sealed my fluid cells, which are hollows ground out 

 in the glass slip, containing mounted Rotifers, as follows: 

 — First I clqLse the cell with the gum daraar and gold 

 size cement, which fixes the cover-glass firmly to tlie 

 slide : when this is dry I put on a ring of the pure gold 

 size, and when that is dry a third ring of Ward's brown 

 cement, taking care that each succeeding coat slightly 

 overlaps the previous ring. 



In this way I hope that my slides may remain perma- 

 nent, but it will take years of experience before a defi- 

 nite judgement can be pronounced. 



Recently a friend sent me an old slide, an anatomical 

 preparation, mounted in a deep glass cell in some watery 

 fluid with a small bubble in it, which he said was at least 

 forty-five years old. He could give me no further infor- 

 mation, except that the bubble had always been there, 

 and had most probably been left in the mount intention- 

 ally to act as a safety valve against the expansion of the 

 fluid. Here, then, is a fluid mount which can be called 

 permanent, and I therefore tried to find out who the 

 mounter was and how it was closed. It has two coats of 

 cement, first a yellow cement like gold size, and over that 

 some black alcohol cement. Mr. W. Sufl"olk, gave me 

 full imforhiation on this slide, and valuable suggestions 

 regarding fluid mounts in general. He say : 



"The slide you send me is by Hett, a well-known 



