Dehydrating and Clearing 99 



pregnated. This series is not reasonable, for there is a much greater and 

 more violent diffusion current when a specimen is passed from water to 

 30 per cent alcohol than there is when a specimen is passed from 70 per 

 cent to 90 per cent alcohol. The author much prefers to use the series of 

 15 per cent alcohol, 40 per cent alcohol, 75 per cent alcohol, and 95 pe r 

 cent alcohol, and would recommend this for the beginning student. This 

 series more nearly represents the intention of the worker, which is to 

 subject the specimen to a graded series of stresses rather than to a graded 

 series of alcohols. 



It is doubtful whether or not it is necessary even to use a series of 

 alcohols when the object to be dehydrated is a thin section attached to 

 a slide. The only purpose of using an intermediate concentration of alcohol 

 between water and 95 per cent is to avoid the rapid dilution of the latter 

 by the water carried over on the surface of the slides. This difficulty can 

 be avoided by using two jars of 95 per cent alcohol. It must be understood, 

 of course, that the section will take just as long to dehydrate as when a 

 series is employed, but one avoids the difficulty of transferring the sec- 

 tion through many jars. 



Other Dehydrants. Within the last decade there have been introduced 

 a number of substitutes for ethyl alcohol, some of them intended for use 

 in circumstances where ethyl alcohol is hard to obtain and others in- 

 tended to serve the purpose of a "universal solvent," miscible alike with 

 water, balsam, and wax. The best of the alcohol substitutes is undoubtedly 

 cellosolve ( ethylene glycol monoethyl ether ) . This substitute has many 

 advantages over ethyl alcohol for purposes of dehydration but cannot 

 be substituted for it in the preparation of many stains and staining solu- 

 tions. It is less volatile than alcohol, so that, if left in an uncovered dish, 

 it does not evaporate so rapidly. It is also somewhat less hygroscopic, so 

 that, under the same circumstances, it does not lose its strength. It has 

 the disadvantage that it is more viscous than alcohol and tends to give 

 rise to greater diffusion stresses; therefore, it must be used in a more ex- 

 tended series of graded mixtures if delicate wholemounts are to be 

 passed through it. However, for the handling of tissues intended for 

 embedding in paraffin and, above all, for the routine handling of tissues 

 in pathological laboratories, cellosolve is strongly recommended. 



The only one of the universal solvents that has found any general 

 acceptance is dioxane (diethylene dioxide). This solvent is readily 

 miscible with water and with balsam, and is slightly less miscible with 

 molten paraffin. Specimens, therefore, can be transferred directly from 

 water to dioxane— a graded series is necessary should they be delicate— 

 and, after having been thoroughly impregnated with dioxane, transferred 

 directly either to the mounting medium or to a bath of molten paraffin 

 for impregnation. In spite of the apparent simplicity of the use of this 



