190 IMBEDDING METHODS. 



It is not possible to state a priori the exact proportion of 

 water that should be added, as this naturally depends on the 

 amount of water already present in the sample of soap taken. 

 In very many cases it will be found that for about 120 grms. 

 of soap solution 5 to 10 grms, of water will be required. 



It is necessary to be very cautious in adding the water, as 

 if too much be taken the mass solidifies more slowly or not at 

 all. The greatest amount of elasticity and consistency is 

 possessed by the mass at the moment in which it contains 

 exactly the minimum amount of water necessary to make it 

 transparent. 



The reasons for this process are explained as follows : Stearate of soda 

 soap is soluble in divers proportions in warm alcohol. On cooling, the solu- 

 tion either solidifies into a homogeneous and pellucid mass, or into a white 

 granular mass ; or, in certain cases, does not solidify at all. The result in 

 each case depends on the proportion of water present in the solution. For 

 instance, if 5 to 6 parts of a tolerably dry soap be dissolved in 100 parts of 

 96 per cent, alcohol, a solution is generally obtained that solidifies into a 

 transparent mass. But such a mass is too soft, and its melting-point too 

 low ; it melts by the heat of the finger. If now, in order to get a harder 

 mass, you add more soap, you will get a solution that solidifies on cooling 

 into a white granular mass ; and it is only after adding to it a certain 

 (small) quantity of water that you will obtain a solution that solidifies on 

 cooling into a transparent mass. If you add more water than is just 

 absolutely necessary to this end the mass will have too high a melting-point, 

 and will solidify more slowly ; and if still more water be added the solution 

 will not solidify for hours, or, indeed, not at all. The more soap you have 

 in your alcoholic solution the more water must you add in order to get a 

 transparent mass, and the more may you add without depriving the solution 

 of the faculty of solidifying. Besides the mass prepared in the proportions 

 given above, useful masses may be made for certain purposes with 10, 20, 

 30, 40 per cent., or more or less of soap in alcohol. Weisker has employed 

 a mass composed of about equal parts by weight of soap and alcohol. 

 Such a mass is transparent, but yellow and oily, and takes a long time to 

 solidify. When cool it is very tough. It requires a considerable tempera- 

 ture to liquefy it, and has less penetrating power than the more alcoholic 

 masses. It is, however, very suitable for hard, and especially for chitinous 

 structures. 



The mass recommended above boils at about 60 to 70 C. 

 Objects should be imbedded in it in a watch-glass or in paper 

 cases in the usual way. Whilst cutting, wet the knife and 

 the mass with strong alcohol (one advantage of this method 

 is that the knife remains perfectly clean). The sections are 

 brought into 96 per cent, alcohol, which frees them from 





