INTRODUCTION. 41 



a time. For the deeper cells marine-glue answers extremely 

 well, provided it is not too soft. It must be melted and 

 dropped upon the slip of glass, then flattened, whilst warm, 

 with a piece of wet glass, and what is superfluous cut away 

 with a knife, so as to leave only the walls of the cell ; these, 

 if they have become loosened, may be made firm again by 

 wanning the under surface of the slip of glass. The surface 

 of the cells must be made quite flat, which can be easily 

 done by rubbing them upon a wet piece of smooth marble 

 covered with the finest emery powder. 



" When about to mount a preparation, a very thin layer 

 of gold-size must be put upon the wall of the cell as well as 

 on the edge of the piece of thin glass which is to cover it ; 

 before this is quite dry, the fluid with the object is to be put 

 into the cell, and the cover of thin glass slowly laid upon it, 

 beginning at one end : gentle pressure must then be used to 

 squeeze out the superfluous fluid, and, after carefully wiping 

 the slide dry, a thin coat of gold-size should be applied round 

 the edge of the cell, and a second coat so soon as the first is 

 dry : a thin coat or two of black sealing-wax varnish may 

 then be put on with advantage, in order to prevent effectually 

 the admission of air into the cell or the escape of fluid out 

 of it. 



" I at first mounted objects for the microscope without 

 enclosing them in a cell previously prepared for their recep- 

 tion, but merely by laying them on the slip of glass with a 

 drop of the fluid, and then covering them up with a piece of 

 thin glass or talc, and afterwards surrounding the latter with 

 a border of thick gold-size, in order to prevent the evapo- 

 ration of the enclosed fluid. Preparations so made will fre- 

 quently last some considerable time ; but eventually the 

 contraction, as it becomes dry, of the outer surface of gold- 

 size forces the remainder, which still continues soft, between 

 the two glasses, and the mounted object is thus injured. I 

 found the same thing frequently to occur when the cells 

 were made of gold-size only without the litharge ; but this 



