TRANSPARENT SOAP. 155 



284. Transparent Soap (KADYI, Zool. Anz., 37, 1879, vol. ii, 

 p. 477). Twenty -five grammes of shavings of stearate of soda 

 soap (any stearate of soda soap will do, but the most to be^ 

 recommended is the sort known in commerce as "weisse 

 Wachskernseife ") are heated in a retort with 100 c. c. of 96 

 per cent, alcohol over a water-bath until the whole is dissolved. 

 Filter if necessary. If a drop of the solution be now poured 

 into a watch-glass it will be seen that it almost immediately 

 solidifies into a white mass. This is not what is wanted, and 

 is a sign that the solution does not contain water enough. 

 Small quantities of water are therefore added by degrees to 

 the solution, and the effect tested from time to time by pouring 

 a drop of the mixture into a watch-glass. The mass will be 

 seen to become more and more pellucid until a point is reached 

 at which it is almost perfectly transparent, with merely the 

 slightest blue opalescence. The preparation of the mass is 

 then complete. 



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 g. 

 soap solution, 5 to 10 g. 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 low a melting point, and will 

 solidify more slowly ; and if still more water be added the solution will not 



