70 SCIENTIFIC AMUSEMENTS. 



solving all the solid matter, and therefore the surplus separates in 

 crystals. Mixed fluids such as these often show the phenomenon 

 of supersaturation to some extent ; crystals not appearing all at 

 once on mixing strong alcohol with the watery solution of the 

 dissolved body, but depositing on standing, especially if the fluid is 

 stirred well. 



Some solids are more easily dissolved by strong alcohol than by 

 water or weak alcohol, so that by adding water to a concentrated 

 alcoholic solution of the substance a " precipitation " of crystals is 

 brought about, because the resulting weak spirit is unable to dis- 

 solve the whole. Camphor forms a good illustration of this ; if 

 some crushed camphor be placed in a bottle with a little nearly 

 absolute alcohol, and the bottle corked and shaken from time to 

 time, a " tincture of camphor " or " camphorated spirit " is pro- 

 duced as the camphor dissolves ; if a few drops of this be poured 

 into a little water, and the whole shaken up, the camphor separates 

 again as a crystalline solid. 



In the manufacture of ordinary hard washing-soap, fatty and oily 

 matters of various kinds (tallow, lard, palm oil, olive oil, cocoanut 

 oil, and such like) are boiled with solution of caustic soda, whereby 

 a chemical change is produced, the sweet liquid termed glycerine 

 being formed, togetherwith soap (Expt. 276); both of these products 

 dissolve in hot water. In order to separate the soap from the water 

 and glycerine, advantage is taken of the fact that soap will not 

 dissolve easily in brine ; so that by adding some strong brine to the 

 liquor (or by throwing a little solid salt in, which comes to the 

 same thing, as the salt dissolves and forms brine), the soap is 

 rendered insoluble (just as the camphor is thrown out of solu- 

 tion in alcohol by adding water), and separates as a soft pasty 

 mass, which floats up to the top of the brine ; this is then 

 skimmed off whilst hot, and poured into large moulds, in which 

 it concretes on cooling to blocks of solid soap, which are then 

 cut up into bars and tablets. The glycerine is not thrown out of 

 solution in this way, but is retained dissolved in the brine, from 

 which it is separated by boiling down and allowing the salt 

 to crystallise out, thus leaving impure glycerine, which is sub- 

 sequently purified by distillation and other processes (Expt. 

 277). 



Expt. 65. To separate Soap from Solution. Cut up a quarter 

 of an ounce of ordinary yellow soap into small fragments, and place 

 them in a flask with a cupful of distilled water ; heat the whole 

 gently without boiling, with occasional shaking ; by and by the soap 

 will dissolve up to a liquid, which will be more or less clear accord- 

 ing to the nature of the soap. Now throw in a tablespoonful of 



