1899] MICROSCOPICAL JOURNAL. 11 



special knowledge and experience in this direction will 

 communicate their experience. Owing to the failure of 

 the cement to keep in the water, I have had to remount 

 over three hundred slides of Rotifers. I have a few deep 

 cells filled with fluid which have kept very well for four 

 or five years, but it appears the greatest difficulty is met 

 with in small shallow cells, which contain only a fraction 

 of a drop of water. 



I have continued my search for a cement which will ab- 

 solutely prevent the evaporation of water. It seems 

 that all cements containing solid particles become porous, 

 or at least slightly pervious to watery vapor, when once 

 quite dry, and therefore it appears to me that one of the 

 objects to he aimed at is to prevent the total evaporation 

 of the solvent. 



A cement that has been specially recommended is a 

 mixture of two third parts of gum damar dissolved in 

 benzole, and one third coachbuilder's gold size. This has 

 undoubtedly very good qualities ; it flows readily from the 

 brush, is very adhesive to glass, and makes a perfect 

 joint between the cover-glass and slide. The water or so- 

 lution of formalin in the cell has no effect upon it. On 

 the other hand I have been warned against the use of gum 

 damar, because when its solvent is all evaporated it can 

 be scraped off as a white powder; but the admixture of 

 gold size modifies this property considerably. Gold size 

 alone has long been known as a good cement, and I have 

 been recommended by a maker to use the variety known 

 in the trade as extra stout, which is not so liable to run in, 

 and which never becomes absolutely hard and dry, and 

 when scraped comes off in strings. Pure gold size, how- 

 ever, does not adhere so strongly to glass as when mixed 

 with gum damar. The solvents of gum damar and gold 

 size are benzole and turpentine, and it has been suggested 

 to me that the evaporation of the last trace of these sol- 

 vents might be prevented by covering the gold size with 



