After cooling, the dry soap is weighed, and the loss of weight
represents the amount of moisture. I have known cases where soap
containing about 83 per cent. of water has been sold at the full market
price. Some soaps also contain more alkali than is actually combined
with the fatty acids of the soap, and that excess alkali is injurious in
washing silks and scouring wool, and is also not good for the skin. The
presence of this free or excess alkali can be at once detected by
rubbing a little phenolphthalein solution on to the freshly-cut surface
of a piece of soap; if free alkali be present, a red colour will be
produced.
LECTURE VII
SHELLAC, WOOD SPIRIT, AND THE STIFFENING AND PROOFING PROCESS
_Shellac._--The resin tribe, of which shellac is a member, comprises
vegetable products of a certain degree of similarity. They are mostly
solid, glassy-looking substances insoluble in water, but soluble in
alcohol and wood spirit. In many cases the alcoholic solutions show an
acid reaction. The resins are partly soluble in alkalis, with formation
of a kind of alkali salts which we may call resin-soaps.
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