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FLUBD MOUNTING. 



By E. D. Evens. 



(Raad December I4th, 1920.) 



It is generally recognised by microscopists that fluid mounts 

 are uncertain. Occasionally a few stand good for many years, 

 particularly those in which the preserving medium is glycerine ; 

 but the majority of aqueous mounts develop air bubbles which 

 gradually enlarge and the object is spoilt. 



If a fluid mount which has been put up for a year or two be 

 held in the hand and the reflection of a bright object in the 

 cover-glass examined, it will generally be found to be distorted, 

 the cover-glass presenting a concave surface. This shows that the 

 liquid must be escaping and as a consequence the internal pressure 

 in the cell has fallen below that of the atmosphere. When the 

 internal pressure reaches the vapour pressure of the medium a 

 bubble will form, a bubble, for instance, of water-vapour, con- 

 taining any dissolved air which may have been originally in the 

 liquid. 



This loss of liquid can only occur by its absorption in the cement 

 of the cell and ring and by its evaporation from this into the air. 

 Water is slightly soluble in most oils and varnishes, as much 

 as 5 per cent, being absorbed in many cases {vide Dr. R. Gr. 

 Morrell, Oil and Colour Chemists^ Ass., February 12th, 1920). 

 Hence the ring of varnish acts like a semipermeable membrane, 

 absorbing the water on one side and evaporating it on the other. 



If this be the true explanation of " leakage " it is obvious that 

 it is necessary, in order to prevent it, to find some sealing material 

 in which water is insoluble or nearly so. If this material is not 

 mechanically strong it is unimportant, as, after sealing, other 

 cements can be put on to give the necessary durability. 



Of all the cements on the market, gold-size, brown cement, 

 Bell's cement, etc., none will stand a week's immersion in pure 

 water even when quite hardened. Shellac, brown cement, 

 etc., become elastic and lose their hold on the glass and can be 

 peeled ofl as a sheet by placing a knife under one edge. Gold- 

 size and Brunswick black, on the contrary, become extremely 

 brittle, and flake off when touched. 



J. Sinel {Outline of the Natural History of our Shores, page 305) 

 recommends a mixture of vulcanised rubber and parafiin wax 



