DEHYDRATING AND CLEARING 33 



The exact proportion of the ingredients varies somewhat according to the 

 opinion of the worker, that given above being the most usual. Specimens may 

 be taken directly from 50 per cent alcohol into this reagent which, however, 

 must be removed thoroughly by washing in pure xylene before the specimen 

 is transferred to wax. It should be pointed out to the inexperienced student 

 that phenol is capable of giving a most unpleasant burn and that carbolxylene 

 itself should be kept away from the hands under all circumstances. This 

 reagent is also useful when one is endeavoring to clear objects in hot humid 

 weather in summer. The alcohol used for the dehydration absorbs water so 

 rapidly under these circumstances that it is almost impossible to clear satis- 

 factorily in xylene alone. 



In the case of small objects, no special precautions need to be observed in 

 the technique of dehydrating. That is, one may merely place the object at the 

 bottom of the tube, fill the tube with the required strength of alcohol, and 

 change this for stronger alcohol as often as becomes necessary. This technique, 

 however, cannot be employed if one is dealing with large objects, since there 

 is a tendency for the water abstracted from the object to accumulate at the 

 bottom of the tube and thus prevent satisfactory dehydration. Objects of a 

 size larger than a grain of corn should always be suspended from the top of 

 the vessel containing the dehydrating agent either in a small bag of cloth or, 

 where the nature of the object permits, from a hook inserted into the cap of 

 the tube or jar. These remarks do not apply when one is dehydrating with 

 cellosolve, which is denser than water. 



The technique of clearing specimens in a hydrocarbon or in chloroform 

 before embedding in wax is exactly the same as dehydrating, and no special 

 precautions need be observed. In the case of delicate objects intended for 

 wholemounting, however, it is necessary to provide some gradient between 

 the alcohol and the essential oil if one is to avoid distortion. The simplest 

 way of doing this is to use a flotation method similar to that used by a bar- 

 tender when he makes a "pousse-cafe." That is, one, first of all, pours a layer 

 of the oil on the bottom of the tube and then very carefully, with the utmost 

 precautions against mixing, floats a layer of alcohol on top of this. The object 

 is transferred from the alcohol to the upper layer of alcohol in the tube through 

 which it drops to the junction of the two fluids. As soon, however, as it has be- 

 come partially impregnated with the clearing agent, it sinks to the bottom of the 

 tube. As it lies there, columns of alcohol will be seen rising from it. When the 

 alcohol has ceased to rise, the object is extracted with a pipette with care so as 

 not to get any alcohol into the pipette, and then transferred to a tube or dish of 

 the pure essential oil where clearing will be completed in a few minutes. 



