130 



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 preserved 

 in glycerine, which were ringed with Miller's caoutchouc cement, 

 which seemed quite good. Mr. Morland'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, espe- 

 cially such materials as gold size. Hollis's glue was good, and 

 he believed this was made of caovitchouc dissolved in ivoocl naphtha, 

 not mineral naphtha. 



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 preparations, 

 and none of these had ever burst through the expansion of the 

 fluid. Another thing worth mentioning in connection with this 

 >^ubject was that he had been told by a mounter always to make 

 the cover-glass a little concave, and then the changes of tem- 

 perature would not be so likely to lift the cover when the fluid 

 expanded, as the cover-glass would be able to spring sufficiently 

 to resist the pressure. 



Mr. Hinton said all those which he had seen of this kind 

 had an air bubble in them, but he did not think they were any 

 the worse for this ; no doubt it allowed of the expansion and 

 contraction of the fluid in the cell. 



The President thought they had had a practically useful 

 discussion. About thirty years ago he belonged to a small micro- 

 scopical society at Manchester, and the members used to practice 

 mounting objects ; but they got into very considerable difficulties 

 with regard to aqueous fluids. Some few of their members, 

 especially one man, however, seemed to have no trouble, and 

 . specimens which he mounted then were as good as ever at the 

 present time. He thought that personal equation came in very 

 considerably into the matter — one man failed whilst another 

 seemed to know exactly how to do it without knowing why. 



