July 12, 1894] 



NA TURE 



259 



and one of Chevieul's greatest discoveries was that in tallow — 

 the fat of oxen or sheep — -you had a salt of organic origin, from 

 I which by decomposing the lallow with heated steam, you could 

 ! obtain the sweet viscous liquid "glycerine," which played the 

 part of base in the compound, and two acidulous compounds — • 

 one a lustrous white wax, called stearic acid, and the other an 

 oil called oleic acid. 

 I Now a salt can have its base replaced by another base. If I 

 , take two solutims, the one containing sulphate of copper, and 

 the other chloride of iron, and add to each sodic hydrate, de- 

 composition takes place in each case, sodic sulphate is left in 

 solution, and the hydrates of copper and iron being insoluble in 

 water, separate out as precipitates. 



In the same way, if we .ndd sodic hydrate to tallow, glycerine 

 separates out, and two salts — sodic oleate and sodic stearate — 

 are formed, a process which we call saponification, as the two 

 sodium salts are " soaps." 



It is not necessary to use tallow ; any vegetable or animal fat 

 or oil will give reactions of a similar character, and it may be 

 broadly staled that soap is formed by the action of sodic or 

 potassic hydrate upon fats or oils which contain fatty acids. 



Organic salt. Bass. Salt. 



Tallow + Sodic hydrate = Soap + Glycerine. 



It is only potassic and sodic hydrates which can be u"ed for 

 ordinary soap-making, as the soaps formed by the combination 

 I of other metallic hydrates with the fatty acids are insoluble in 

 water, and therefore useless for detergent purposes. 



The soap formed by using sodic hydrate has the property of 



■ setting hard, and all the ordinary forms of washing-soap contain 

 ; sodium as the base ; the p'ltash soaps are far softer, and do not 



set, the soft soap used for scrubbing and cleansing in many 

 manui'acturing processes, and also a few toilet creams and 

 shaving pastes, being of this character. 



It would occupy far too much time, and would, moreover, be 

 outside the scojie of this lecture, to go into the details of the 

 manufacturing methods by which 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 con- 

 sists 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 calcic carbonate or chalk, which being insoluble sinks 

 as a mud to the bottom of the vessel, whilst sodic hydrate 

 remains in solution. 



Calcic hydrate 1 _ f Sodic hydrate 

 Sodic carbonate J ~ \ Calcic carbonate. 

 Of late years the soap-boilers have to a great extent bought the 

 sodic hydrate direct from the manufacturer, and so have 

 avoided this operation. 



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

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

 and as the conversion into soap proceeds, so stronger leys are 

 used until the whole of the fatty matter has been saponified. 



If a strong ley had been used at first, the soap as it formed 

 being insoluble 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 

 j solution is suflicient'y strong, the soap will on standing separate 

 as a fluid layer on the surface of the spent ley, which contains 

 the glycerine set free during the saponification, but in any case 

 'separation can be rapidly brought about by adding salt to the 

 1 liquid, when the soap being insoluble in salt water or brine, 



■ separates out and is removed and placed in moulds to harden. 

 1 The block of soap so cast is then cut first into slabs, and then 

 'again into bars. 



1 A soap made in this way with tallow or lard as the fatty 

 matter, would be "white curd," whilst if yellow bar is 

 Irequired, rosin is added to the mixture of ley and soap after 

 I most of the f.it 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 

 ingredients are of groat purity, the product goes by the name of 

 " Primrose " soap. Bar soaps so made on a large scale are, as a 

 rule, the stock from which the various forms of toilet soap are 



NO. 1289, VOL. 50] 



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, whilst cheap toilet soaps, especially cheap 

 transparent ^oaps, should be studiously avoided. 



The demand made by consumers for cheap soaps, which in 

 many cases are sold retail at prices considerably below the 

 wholesale market price for a true soap, has given rise to the 

 introduction of highly watered soaps, caused to set hard by the 

 addition during manufacture of sodic sulphate, which enablis 

 the manufacturer to make a so-called soap often containing less 

 than 20 per cent, of true soap. 



Any person desiring to obtain the fullest particulars as to the 

 manufacturing details of the soap trade, cannot do better than 

 consult Dr. C. R. Alder Wright's admirable treatise upon the 

 subject. 



Having got our soap, the next point is to try and gain an idea 

 of the way in which it acts as a detergent. 



Supposing we are fcrrtunale enough to have a sample of pure 

 neutral soap, we find that on dissolving some of it in water, it 

 undergoes a partial decomposition into alkali, and fatty acid, 

 this action being called the hydrolysis of soap. 



The small quantity of alkali so set free, attacks the fatty 

 matter which glues the dirt to the skin, and by dissolving it 

 loosens and enables the water to wash off the particles of dirt. 



If this were the only action, however, soap would have no ad- 

 vantage over soda, a solution of which would equally well per- 

 form this part of the operation. As the soap decomposes and 

 the alkali removes the grease and dirt, the fatty acid liberated 

 simultaneously from the soap comes in contact with the newly- 

 cleansed skin, and not only softens and smooths it, but also 

 neutralises any trace of free alkali, and so prevents irritation and 

 reddening of the cuticle. 



These are protably the main actions by which soap cleanses, 

 but other causes also play a subsidiary part. W'e know that a 

 solution of saap causes a lather when a.;iiated, this being due to 

 the cohesive power given to the particles of which the liquid is 

 built up by the presence of the soap, a phenomenon which 

 also enables us to blow Imbbles with the soip solution on ac- 

 ! c lunt of the strength of the fine film of liquid, a property which 

 is not found in water alone. 



The power of cohesion which the soap solution possesses is in 

 ] all probability an important factor in removing the particles of 

 I dirt from the skin at the moment that they are loosened by the 

 action of the alkali. Prof. W. Stanley Jevons suggested yet a 

 fourth way in which the soap solution might act ; when finely 

 divided clay is suspended in water, the microscope reveals the 

 fact that the minute particles are in rapid movement, and hence 

 settle but slowly in the liquid. This movement he christened 

 pedetic action, and he observed that the addition of soap or 

 silicate of soda — often used in soap — to the liquid, enormously 

 increased this agitation of the particles, which would tend to aid 

 the breaking away of the dirt particles the moment they were 

 set free. 



Many soaps, even .imong the varieties intended for the toilet, 

 contain a considerable excess of free alkali, which being greater 

 than the liberated fatly acids can neutralise, cause most painful 

 irritation of the skin, as is testified to by the smarting which 

 I annoys the chin after the use of certain shaving soaps ; and every 

 lady knows that an alkaline soap, when used for washing the 

 hair, renders it harsh and brittle, and destroys the gloss, but in 

 both cases a rapid rinse with water, containing a few drops of 

 vinegar, will neutralise the free alkali and prevent much of the 

 mischief. 



We have now dealt with our grease solvents and dirt 

 looseners, but without the aid of water they would be use- 

 less ; and experience teaches us that the source of the water 

 used for cleansing, has a great deal to do with its efficiency 

 whether used with or without soap. 



As the newborn raindrops fall from the breaking clouds, 

 they are practically pure water, containing at most traces of 

 gaseous impurities which the mist has dissolved from the upper 

 sirata of air whilst journeying in the form of cloud, and where 

 the rain is collected in the open country, it gives us the purest 

 form of natural water, healthful to drink, because it is highly 

 aerated, and free from all impurity, organic and inorganic, ami 

 delightful to wash in because of its softness and the ease with 

 which the soap gives a lather. 



In towns, however,a very different state of thiogs exists, as the 



