BASED UPOX EXPERIMENTS WITH THE BLOW-FLY. 345 



the delicate tissue of the fly. For this reason, a few years ago, 

 I set myself to inquire whether some as yet untried combination 

 might not be more satisfactory. I found that a compound, such 

 as had been used by many before me, of paraffin (melting point 

 45° C.) 60 grains and white wax 30 grains was a good basis to build 

 upon. This mixture alone, though it did not form vacuoles, had a 

 tendency to crack on cooling ; but the addition of exceedingly 

 small quantities of certain other materials quite prevented this. 

 Thus, one or two minims of a weak solution of caoutchouc was 

 perfectly effectual. Three grains of camphor produced the same 

 result. For most of the preparations I have exhibited here, the 

 mixture of paraffin, wax, and caoutchouc, has been employed, 

 being the more easily prepared. 



The melting point of both these mixtures is 48° C. I always 

 cooled them slowly, simply putting the containing capsules on a 

 wooden table at the temperature of the laboratory, which I 

 generally kept at about 16° C, and I never found them form 

 vacuoles. My object was to get a good medium, suitable for 

 cutting at this temperature ; and for a long time I felt fairly 

 satisfied. Nevertheless I could not help observing that some- 

 times the imbedded insect was too hard, causing the sections to 

 tear in cutting ; so I reduced the quantity of wax and added a 

 little creasote, and my most recent specimens, which are much 

 better than my earlier ones, have been imbedded in a material of 

 which the following is the composition : — 



Paraffin (45° C.) . . . . . .80 grains. 



White wax . . . . . , . 10 „ 



Anhydrous creasote ...... 2 minims. 



Solution of caoutchouc in pure benzol (1 gr.-to 



5 fl. drachms) . . . . . . * 2 ,, 



This combination has all the advantages of those mentioned 

 above, without the drawback of being too hard, and its melting 

 point is 3° lower, viz., 45° C. 



3. Stretching, Fixing, and Mounting, — My sections, which are 

 all made with the Cambridge rocking microtome, lose about g of 

 the width of the block, from which they are cut, in consequence of 

 wrinkles or compression from the impact of the knife. Professor 

 Lee has told us how to obliterate the wrinkles and restore the 



