126 CYTOLOGICAL TECHNIQUE 



dehydration. The usual series provides a good compromise. The 

 dehydrated section is usually transferred to the solvent contained in 

 the mounting medium, and from this to the mounting medium 

 itself. Some hydrophobe media, however, contain no solvent. 

 Methyl salicylate is an example. The tissue is transferred directly 

 from absolute ethanol to such media. 



Both hydrophil and hydrophobe mounting media are further 

 subdivisible into those that are adhesive (and thus bind the cover- 

 slip to the slide) and those that are not. If a preparation is to be 

 kept permanently, it is almost essential that the coverslip should 

 be bound to the slide. The easiest way of achieving this is to use 

 an adhesive mounting medium. 



If perfect apposition can be obtained between two solid objects, 

 they will ordinarily adhere, because their molecules will be drawn 

 towards one another by the same forces that hold the molecules 

 together in the objects themselves.^^ There is no question, how- 

 ever, of causing coverslip and slide to adhere in this way, partly 

 because their surfaces cannot be made flat enough for perfect 

 apposition on the molecular scale, partly because a section or 

 other microscopical object must necessarily intervene. Some de- 

 gree of adhesion will be achieved if a fluid intervenes, provided 

 that the fluid is able to wet both the surfaces concerned.^^ It is 

 a fortunate circumstance that glass can be wetted both by water 

 and by the solvents used in hydrophobe mounting media. The 

 adhesion will be very poor, however, unless the liquid not only 

 adheres to the glass but also coheres within itself. In other words, 

 it must be viscous, or else be capable of actual conversion into 

 a solid. The presence of long molecules in the liquid will thus 

 favour adhesion. So that the surfaces may be easily wetted, it is 

 desirable that the adhesive should be at first of fairly low vis- 

 cosity. The necessary increase in viscosity or conversion to solid 

 may be achieved either by cooling or by the evaporation of a 

 solvent. 



The adhesive hydrophobe mounting media, such as DPX 

 (p. 131), are hardened by evaporation of the solvent, which is 

 usually xylene. The commercial product that is sold under this 

 name is a mixture of three isomeric dimethyl benzenes with ethyl 



