250 THE AMERICAIsr M0:NTHLY [November, 



satisfactory. Having treated of those cells most usually adopted, we 

 are novv^ ready to pass to the various cements employed, both for fasten- 

 ing the cells to the slide as well as the cover-glass to the cells. 



Marine Glue. — Nothing can surpass the utility this cement presents, 

 whether in the tenacity with which it holds, or the readiness with which 

 the cells or troughs for the examination of aquatic life can be constructed ; 

 it is the most suitable and convenient the microscopist can employ, as, 

 by its aid, he can join glass and metal in any variety and form his 

 requirements may suggest. To melt it gentle warmth must be used, 

 but the temperature must not be sufficient to burn it, or its adhesive 

 properties are destroyed. However valuable it may be for the purposes 

 here indicated, it is not adapted for cementing cover-glasses. For that 

 we require a varnish which can be put on without heat, and of this 

 kind we have a variety to choose from. Whatever varnish we use 

 as a cement must not be too thin. It ought to be of the consis- 

 tency of syrup, or that of the densest and freshest glycerine. If it is 

 firmer than that, there is a difficulty in sealing up a preparation so that 

 the joint is solid and free from leaks. If it is thinner than that, there is 

 the danger of its running in ; but if it is of about the firmness indicated, 

 these liabilities are greatly lessened, if not entirely abolished. 



Gold Size. — This is by far the best cement for this purpose, for a 

 cement should not become hard too quickly. It is better that it should take 

 a day or two after a first thin application, before it is sufficiently set as 

 to allow of a second and thicker coating. This cement answers well 

 for sealing up cells containing any of the preservative fluids but 

 especially for those not containing glycerine. Glycerine is influenced 

 greatly by changes of temperature ancl barometric pressure. It is also 

 hygroscopic, therefore moisture is readily absorbed by it; so, if the 

 cement is cracked by the expansion of the glycerine, moisture is soon 

 absorbed, and, swelling the fluid contents of the cell, causes a general 

 bursting of the cement. The author has found the value in glycerine 

 mounting, and with other preservative fluids as well, of an addition to 

 the gold size of about one-third solution of India-rubber in benzole. 

 This gives an elasticity to the gold size, which seems to allow of a 

 small amount of expansion without cracking the cement. The older 

 the gold size the more reliable it is as a cement. 



Gum Da?nmar Varnish. — This is a clear, tenacious cement, which 

 may be readily made of any consistency desirable. By putting clear 

 pieces of the gum into benzole, and, when dissolved, letting the benzole 

 evaporate till the fluid is of the density required, it may be decanted off' 

 into small bottles for use. It dries very readily, and a thin coating put 

 round a cell is sufficiently hard in a couple of hours to receive an 

 additional one. There are several other cements more or less recom- 

 mended, but the student will find in these two all that he needs to meet 

 his requirements. 



Mounting Transparent Objects. — Preservative Media. — These 

 are of two kinds — the solid, such as Canada balsam and glycerine 

 jelly; and the fluid, as glycerine. Canada balsam, while undoubtedly 

 that which takes the first place, especially with novices, in the art of 

 mounting, on account of the apparent simplicity in its use and the 

 readiness with which a slide can be prepared by its means, has its 

 drawbacks ; and perhaps the first which the student will experience is 



