16 THE AMERICAN MONTHLY [Jan 



ways seemed to him that they should keep in mind the two 

 great principles involved in the use of any cement — that it 

 was wanted to secure the contents against evaporation, 

 and to prevent the cover from moving-. His experience 

 was that they must prevent by all means the escape of the 

 last trace of solvent from the cement employed, whether 

 the object was mounted in balsam or in fluid, and to ensure 

 his the best way was to cover the cement with another 

 which was dissolved in a different solvent — gum damar 

 was a good material when the solvent was benzole, and a 

 spirit cement over that would prevent the escape of the 

 last trace of benzole and be perfectly durable. He has 

 some slides which were cemented with gum damar and 

 gold size overlaid with shellac and spirit cement, and these 

 had remained good after a number of years. He had also 

 some which had been sealed with Brown's cement, but 

 this had failed. The first requisite was to get something 

 which would not be acted upon by the fluid in the cell ; and 

 the next was to give this an overlapping layer of some 

 other cement which was not acted upon by the solvent 

 of the first. 



Mr. Karop said he had a series of slides of Tipula pre- 

 served in glycerine, which were ringed with Miller's 

 caoutchouc cement, which seemed quite good. Mr. Mor- 

 land's idea was of course very good, but it would, he 

 thought, be found rather difficult to make one's own cement 

 in the small quantities required, especially such materials 

 as gold size. Hollis's glue was good, and he believed this 

 was made of caoutchouc dissolved in wood naptha, not min- 

 eral naptha. 



Mr. Earland said he had recently received three slides, 

 dated 1851, containing desmids, and the fluid in these was 

 intact ; they were sealed with what looked like gold size. 



Mr. Nelson said he had some very large insect prepara- 

 tions, and none of these had ever burst through the expan- 

 sion of the fluid. Another thing worth mentioning in con- 

 nection with this subject was that he had been told by a 

 mounter always to make the cover-glass a little concave, 

 and then the changes of temperature would not be so likely 



