36 MATERIALS AND PROCESSES OF SLIDE-MAKING 



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 minutes. 



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

 sistency, so that one may handle a slide made with it within half an hour. 

 A slide made in Farrants' 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: 



Berlese's Medium: 



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 with care so as not to include too many air bubbles. 

 When solution of the gum acacia is complete, the chloral hydrate is added 

 and stirred to solution. 



This mixture suffers from the same disadvantage as does Farrants' in that it 

 is difficult to secure clear gum acacia. There is, however, no other medium of 

 such high refractive index which is so 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 appen- 

 dages well displayed for examination. If difficulty is encountered in drying the 

 medium, it is probably due to the very large quantity of chloral hydrate. In 

 those laboratories where the atmosphere is commonly humid, it would be well 

 to reduce the quantity of chloral hydrate to 60 Gm. 



There are two main types of resinous mounting media: (1) Those into 

 which the object may be placed directly from alcohol and (2) those for which 

 the object first must be cleared. The former type of medium is commonly 

 referred to as a "neutral" mountant and is used almost exclusively for mount- 

 ing stained blood films which are very sensitive to the acid which inevitably 

 develops in balsam mounts. The best known of these neutral mountants is a 

 proprietary compound of secret composition known as euparal. This may be 

 obtained commercially, but for those who prefer to prepare their own solu- 

 tions a very excellent substitute is: 



