COLLECTION OE SPECIMENS 181 



ferred to. Moreover, there would seem to be a 

 tendency for the sealing medium to " run in," 

 unless much care is taken. 



Cements. The question of the material to he 

 used for sealing the edges of the finished cells is 

 a matter of some importance. I therefore add a 

 few notes giving my experience of the various 

 cements that I have tried. Gold size is so 

 strongly recommended in some of the hooks that 

 I hesitate to throw any doubt upon its efficacy : 

 my acquaintance with it, however, has not been 

 favourable, for, unless the film of jelly in the cell 

 is very thin, I have almost always found that, 

 sooner or later, the size " runs in," colouring the 

 jelly a much deeper shade than its ordinary tint. 

 Canada balsam is a very good cement, but it is 

 apt to crack and chip off when old ; and the same 

 may be said of Asphalt varnish when prepared 

 in the more usual way by dissolving asphalt in 

 spirits of turpentine. If made, however, according 

 to the recipe given in Davies' "On Mounting 

 Microscopic Objects," I have found it much 

 more reliable. This recipe is as follows : Take 

 J drachm of pure indiarubber, 2J ounces (fluid) 

 of mineral naphtha, and 1 ounce of pure asphal- 

 tum ; dissolve the indiarubber in the naphtha, 

 placing the bottle in hot water if necessary for 

 the purpose ; then dissolve the asphaltum by the 

 like means. 



By far the most useful sealing medium, to my 



