130 CYTOLOGICAL TECHNIQUE 



Africa. The gum is taken from both wild and cultivated trees.* 

 Its name refers to the fact that it was first imported into Europe 

 from Arabian ports. The gum consists of the calcium salt of 

 arable acid,^°^ which owes its acidity to the carboxyl group of 

 glucuronic acid. Hydrolysis yields arabinose, rhamnose, and 



HoCOH COOH 



O 



■ CHi 



arabinose rhamnose galactose glucuronic acid 



Skeleton-formulae of the hydrolytic products of gum arable 



galactose, in addition to glucuronic acid. The exact way in which 

 these are bound together in the natural product is not known, but 

 the evidence suggests a branched chain, in which the components 

 are not arranged in a regularly repeated order. ^^^ The molecular 

 weight is about 240,000. ^'^^ The shape and size of the molecule 

 determine its high viscosity in aqueous solution. 



The refractive index of the medium made up according to 

 the formula given above is 1-423 (or 1-424 if saturated with 

 camphor).^* It is best to drive off most of the water (and thus 

 increase the refractive index) by leaving the slide overnight in the 

 paraffin oven, after applying the coverslip. The latter adheres 

 firmly. 



It has been stated in print that 'The ordinary media of the 

 Farrants' type have an index of refraction just over 1-3.' ^^ This 

 statement cannot be correct, whichever formula be followed; for 

 the refractive index of water itself is 1-333, while those of the other 

 constituents are very much higher. 



The glycerine in Farrants's medium prevents the gum from 

 cracking when the water is driven off. It also helps to make the 

 tissue transparent, because it readily penetrates the minute spaces 

 in them, while the large molecules of the gum diffuse slowly. The 

 uneven distribution through the tissue of the two substances of 

 high refractive index in this medium is probably the cause of the 

 imperfection of the phase-contrast image. 



Methyl salicylate.^'' Non-adhesive hydrophobe media are very 



