DEHYDRATING AND CLEARING 31 



over on the surface of the sHdes. 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 section through many jars. 



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 intended to serve the purpose of a "uni- 

 versal 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 solutions. 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 extended series of graded mixtures if delicate wholemounts are to be 

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

 ding in paraffin and, above all, for the routine handling of tissues in patho- 

 logical laboratories, cellosolve is strongly recommended. 



The only one of the "universal solvents" which 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 mount- 

 ing medium or to a bath of molten paraffin for impregnation. In spite of the 

 apparent simplicity of the use of this solvent, there are certain great objec- 

 tions. The first of these is the toxicity of dioxane vapor to humans. Dioxane 

 is a cumulative poison and has been shown to affect seriously the function 

 of both the liver and the kidneys. This does not matter very much where it 

 is used by an individual who knows himself to be free from hepatic or renal 

 disorder, particularly where he is exposed only to low concentrations of the 

 vapor for relatively short periods. However, it militates heavily against the use 

 of this reagent in large classes where the instructor is responsible for the 

 health of individuals without knowing their physical idiosyncrasies. Another 

 disadvantage is in the heavy diffusion stresses which are set up when materials 

 are transferred from dioxane to molten paraffin. This is not so important when 

 dealing with tissue blocks for routine histological examination but it is almost 



