SMELLING SALTS are usually either pure ammonia or its carbonate. Take a small piece of burnt unslaked lime, say li oz., and add to it in a mortar 1 oz. of muriate of ammonia, rub them well to gether, and the pungent smell of ammonia cal gas will be given off; then bottle, per fume it, and cork. The chlorine of the sal-ammoniac has a greater affinity for lime than ammonia, it therefore leaves the ammonia and combines with the lime, forming the chloride of calcium, the ammoniacal gas is set at liberty. SOAP. This useful compound is ob tained by the action of alkalies upon oily substances. There are, accordingly, a great variety of soaps ; but those com monly employed may be considered un der the heads of-1, fine white soaps, scented soap, &e. ; 2, coarse household soaps; 3, soft soaps. The materials used in the manufacture- of white soaps are generally olive oil and carbonateof soda: ate toter is 'rendered caustic by the oper ation of quiekliine, and the solution thus obtained is called soap ley: -The oil 'and a weak ley are first boiled together, and portions of stronger ley are gradually added, till the soap, produced by the mu tual action of the oil and alkali, begins to become tenacious, and to separate from the water ; some common salt is then generally added to promote the granula tion and perfect separation of the soap : the fire is then drawn, and the contents of the boiler allowed to remain for some hours at rest, so that the soap may more completely collect. When it is perfect it is put into wooden frames or moulds ; and when stiff enough to be handled, it is cut into oblong slices and dried in an airy room- Perfumes are occasionally added, or various coloring matters stirred in while the soap is semifluid to give it a mottled appearance. The Spanish soap is marbled by stirring into it a solution of sulphate of iron, which is decomposed by the soap, and black oxide of iron se parated in streaks and patches through the mass. The action of the air converts the exterior into red oxide, while the in terior long retains its black color ; hence a slice of this soap presents a black mot tled centre, surrounded by a reddened external layer.
Common household soaps are made chiefly of soda and tallow ,• or if potash is used, a large addition of common salt is made to harden the soap, which it pro bably effects by the transference of soda. Yellow soap has a portion of resin added to it. Soft soaps are .gencrally made with potash instead of soda, and fish oil : it has a tenacious consistence, and appears granulated. Soap is soluble in pure water and in alcohol; the latter solution Tellies when concentrated, and is medi cinally known under the name of opodel doe- When carefully evaporated the soap remains in a gelatinous state, which forms, when dry, the article sold under the name of transparent soap.
The earths and common metallic oxides form insoluble soaps ; and accordingly these are precipitated when earthy and metallic salts are added to solution cf soap. It is the sulphate of Hine and car bonate of lime in common spring water which thus render it unfit for washing, and give it what is termed hardness ; and, upon this principle, a spirituous solution of soap is a simple and valuable test of the fitness of any river or spring water for the purposes of the laundry. if it merely renders the water slightly opal escent, as is the case with rain and other soft waters, it may be used for washing ; but if it become milky, it is usually too hard to be conveniently employed ; and when one was the skin with hard water, the separation of the insoluble calcareous soap is extremely disagreeable; it ad heres to the skin, and soils instead of cleansing it.
The chemical nature of soap has been laboriously examined by Chevreul, who has shown that the alkali in the process of saponification converts the oil into pe culiar acids, as he terms them ; the chain of the oil forming oleic acid, and the stearine nuirgaric acid : so tht soluble soaps are oleates and margarates of soda and potash. Ho has enumerated several other fatty acids similarly produced.