io8 THE POPULAR SCIENCE MONTHLY. 



manufacturing methods by whicli soap is made on the large scale, 

 and if I give a rough idea of the general processes employed it 

 will be sufficient for the purpose. 



Carbonate of soda is first converted into hydrate by dissolving 

 it in water and then boiling with quicklime. Quicklime consists 

 of calcic oxide, and this, when put into the vat containing the 

 sodic carbonate in solution, combines with water, forming calcic 

 hydrate, which then reacts with the sodic carbonate, forming cal- 

 cic carbonate or chalk, which being insoluble sinks as a mud to 

 the bottom of the vessel, while sodic hydrate remains in solution. 



The solution of sodic hydrate, called caustic lye, is made in 

 different strengths, and tallow is first boiled with a weak lye, and 

 as the conversion into soap proceeds, so stronger lyes are used 

 until the whole of the fatty matter has been saponified. If a 

 strong lye had been used at first, the soap as it formed being in- 

 soluble in strong alkalies would have coated the surface of the fat 

 and prevented its complete conversion. 



If at the end of the saponification process the alkaline solution 

 is sufficiently strong, the soap will, on standing, separate as a fluid 

 layer on the surface of the spent lye, which contains the glycerin 

 set free during the saponification ; but in any case separation can 

 be rapidly brought about by adding salt to the liquid, when the 

 soap, being insoluble in salt water or brine, separates out and is 

 removed and placed in molds to harden. The block of soap so 

 cast is then cut first into slabs and then again into bars. A soap 

 made in this way with tallow or lard as the fatty matter would 

 be " white curd," while if yellow bar'is required, rosin is added to 

 the mixture of lye and soap after most of the fat has saponified. 



When ' rosin is boiled with alkaline solutions, a compound is 

 formed by the direct union of the resinous acids with the alkali, 

 which strongly resembles ordinary soap, so that the yellow soap 

 is really a mixture of fatty and rosin soap, and when the ingredi- 

 ents are of great purity the product goes by the name of " prim- 

 rose" soap. Bar soaps so made on a large scale are, as a rule, the 

 stock from which the various forms of toilet soap are made by 

 processes intended to render them more attractive for personal 

 use, but generally the consumer gets far better value for his 

 money, and far less injury to his skin, by using a good " white 

 curd " or " primrose " soap than by employing a high-priced toilet 

 soap, while cheap toilet soaps, especially cheap trans]3arent soaps, 

 should be studiously avoided. 



The demand made by consumers for cheap soaps, which in 

 many cases are sold. retail at prices considerably below the whole- 

 sale market price for a true soap, has given rise to the introduc- 

 tion of highly watered soaps, caused to set hard by the addition 

 during manufacture of sodic sulphate, which enables the manu- 



