Mounts and Mountants 105 



obtain a sample of gum acacia that is not contaminated with pieces of 

 bark, sand, and dirt. A 50 per cent solution of this material, even when 

 diluted with the glycerin, is difficult to filter, and it is better to obtain 

 this medium from a supplier of materials than to make it up oneself. A 

 medium that avoids this difficulty by using synthetic materials is: 



Gray and Wess's Medium: 



First make a smooth paste of the dry alcohol with the acetone. Then 

 mix half (5 ml) of the water with the glycerin and lactic acid and stir 

 this into the paste. Add the remaining 5 ml of water drop by drop, 

 stirring constantly. The mixture thus produced is cloudy at first but will 

 become transparent if heated on a water bath for about 10 min. 



The advantage of this medium is that it sets fairly rapidly to a tough 

 consistency, so that a slide made with it may be handled within half an 

 hour. A slide made in Farrants's medium may require drying for several 

 days before it is safe to stand it on edge. 



The best known of the high-refractive-index, watery media is: 



Mix the water with the acid and the dextrose syrup. Dissolve the gum 

 acacia in this mixture. This will take a week or so, and the material should 

 be stirred at intervals, care being taken not to include too many air 

 bubbles. When solution of the gum acacia is complete, the chloral hy- 

 drate is added and stirred to solution. 



This mixture suffers from the same disadvantage as does Farrants's 

 medium in that it is difficult to obtain clear gum acacia. There is, how- 

 ever, no other medium of such high refractive index that is suitable for 

 mounting small arthropods. The chloral hydrate seems to act as a narcotic, 

 so that a small specimen placed in this medium usually expands into a 

 relaxed condition with all the appendages well displayed for examina- 

 tion. If difficulty is encountered in drying the medium, it is probably due 

 to the very large quantity of chloral hydrate. In those laboratories where 



