228 SCIENTIFIC AMUSEMENTS. 



water to form crystallised gypsum (or sulphate of calcium contain- 

 ing water of crystallisation), the crystals of which stick together 

 as a porous mass, which absorbs the rest of the water in its pores, 

 thus becoming apparently solid in much the same way as the 

 precipitated carbonate of calcium in Expt. 128. The setting of 

 most kinds of cement and mortar is due to a somewhat analogous 

 change of physical structure of the mass, resulting from some kind 

 of chemical action taking place to begin with. 



Expt. 274. To obtain Acids from Sugar. In Expts. 189 to 193 

 we have seen that by fermenting sugar alcohol is obtainable, and that 

 by the souring or oxidation of alcohol under certain conditions acetic 

 acid is formed as vinegar. If sugar be boiled with nitric acid diluted 

 with water for some time a vigorous action is set up ; the nitric acid 

 parts with some of its oxygen, and the sugar is partly converted into 

 carbon dioxide which escapes, and partly into oxalic acid, which can 

 be obtained in the solid state by evaporating the whole to dryness 

 on the steam bath, dissolving the residue in the smallest possible 

 quantity of hot water, and allowing the liquid to crystallise. 



Milk contains a peculiar kind of sugar (milk sugar) different 

 from cane and fruit sugars ; when milk turns sour an acid termed 

 lactic acid is formed, somewhat as acetic acid is from ordinary 

 sugar and fruit sugar. 



Expt. 275. To prepare Soap for Household Use. The differ- 

 ent kinds of soap in the market are all essentially substances 

 prepared by the double decomposition of caustic alkalies and fatty 

 or oily matters, resulting in the formation of soap and glycerine. 

 According as the alkali is potash or soda, so, as a rule, is a soft or 

 a hard soap obtained ; whilst the nature of the fatty matter also 

 influences the character of the soap obtained. Ordinary household 

 and scouring soaps are generally made from very coarse kinds of 

 fatty matter, such as the grease from slaughtered horses, fatty 

 substances from tannery refuse, the "foots" or thick sediments 

 obtained in refining oils, and such like ; toilet soaps are, or at least 

 ought to be, made from superior qualities of fats and oils, subse- 

 quently refined, purified, and scented, and finally worked up into 

 tablets by machinery. 



Dissolve some solid caustic soda in about four times its weight 

 of water, and make the solution hot ; melt down a quantity of 

 kitchen grease or lard mixed with olive oil, taking about six times 

 as much as you have dissolved of caustic soda, or rather more. 

 Pour the strong caustic soda solution in a slow stream into the 

 warm melted fat contained in a convenient basin or pail, stirring 

 vigorously the while; stir for some time till very thoroughly 

 intermixed, and then cover up with a cloth and set by in a warm 



