224 E. D. EVENS ON FLUID MOUNTING. 



gold-size should be applied. I generally use a gold-size containing 

 indiarubber, composed of : 



Gold-size 2 volumes 



3 per cent, rubber in benzene ... 1 volume 



Let this stand in an uncorked bottle until the two have mixed 



perfectly to give a clear brown liquid. The slide should now 



be put away for six months or so to harden o3 and can then be 



finished with a coat of Brunswick black, followed by one of 



gold-size. It is advisable to give it a further coat of shellac after 



two or three years to prevent complete hardening of the gold-size. 



This D.I. P. cement is suitable for dilute formalin, pure or 



dilute glycerine, glycerine jelly, etc. It will also resist saturated 



aqueous potassium mercuric iodide solution. It is, of course, 



not suitable for oily media like paraffin oil, balsam, etc. It 



ought to be satisfactory for strong alcohol, but in this case it 



would be better to omit the gum dammar and replace the two 



shellac coats by two of gum arable containing a little glycerine. 



The argument often used against fluid mounts, viz. that the 



difierence in the expansion of the fluid and the cell under the 



influence of heat will upset any cement ring, is, I believe, false. The 



actual difference is an exceedingly minute quantity and there is 



ample allowance for it in the elasticity of the cover-glass, which acts 



as the stretched membrane of a drum, taking up any small change 



in the volume of the liquid without an undue increase in pressure. 



This method has been in use only for about four years, and 



of course it is too early as yet to pass judgment upon it ; but 



up to now no failures have occurred. 



It is possible that the rubber-paraffin mixture might with ad- 

 vantage be replaced by a guttapercha-paraffin mixture, as water 

 dissolves in guttapercha only to the extent of about 1"5 per cent, 

 while the corresponding figure for rubber is 10-12 per cent. The 

 value of this suggestion, however, can only be tested by experiment. 



[Note. — August 1921. — Since the above was written I have 

 tried this guttapercha-paraffin compound and do not think it 

 appears more promising than the original, as it seems more 

 brittle, I also now think that the gum dammar in the D.I.P. 

 cement may with advantage be reduced to half the value given 

 above, i.e. to two parts, for eight of wax and one of india-rubber.] 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XIV., No. 87, November 1921. 



