Considerable light has been thrown upon the manner of removal of dirt by soap, by the researches upon Pedesis of Prof. W. Stanley Jevons, F.R.S., who has given this name to a micro scopic phenomenon long known as the "Brownian movement " of small particles. When clay is stirred up with water, and the water allowed to stand, it clears itself very slowly, aud microscopic examination showed that this was due to a kind of molecular movement of infinitesimally small particles of the clay. To this movement, Prof. Jevons gave the name Pedetie action (rid. Quarterly Journal of Science ' for April, 1878, No. LVIII.), and he found that it was largely iufluenced by the addition of eertain substanees to the water containing clay in suspension. Soap and silicate of soda enormously inoreased the Pedetio action, or movement of the particles (Report of the British Association for the Advancement of Science, 1878, p. 435), and from observations made by Prof. Jevons, and by the writer of this article (who hoFes to extend his researches in this direction), it seems clear that in the action of these substances in promoting this molecular movement of extremely minute particles, is to be sought part of the explanation of the cleansing power of soap.
There are few things which are so ill understood in practical life as the real value, or, what is the same thing, the proper price of soap. From what has been said, it appears clear that the real value depends upon the amount of dry (anhydrous) soap present, and upon the proportion of stearic, palmitic, and margario acids to oleic, and to that of the coeinie, laurostearic, &c. In other words, the determination of the following elements is necessary to arrive at an estimate of the value of a soap :— (1) The percentage of water ; (2) the percentage of soda available for detergent purposes (a), com bined with the fatty acids, (b) as caustic, carbonate, silicate, or aluminate; (3) the percentage of fatty acids ; (4) the nielting-point of those fatty aeids (see Candles; and Oils, Detection and Analysis, p. 1477).
It .would be very greatly to the advantage of all large eonsumera of amp, aa well aa to soap manufacturers themselves, if soaps were to be sold guaranteed to contain ao much per cent. of fatty matter of a given tnelting-point, eombined with the full quantity of soda necessary for its complete aaponification.
Brief inatructions will now be given for the most suitable methods, consistent with accuracy, for the analysis of Kap ; for fuller information, manuals of technical ehemical analysis should be consulted.
Uniformity of Sample,—Great care should be taken to ensure thia ; since aoap loses water rapidly on exposure, the soap should be sliced up in thin pieces, well shaken, and kept in a well stoppered bottle. Other convenient plane are (1) to weigh out at once all portiona required for analysia, (2) to make a standard solution of the soap, say 100 grm. in 1 /it., and measure off what is required, taking care to avoid loss by evaporation. In analysing, case-haxdened soaps (p. 1783), care must be taken to see that the section of the bar includes a proper proportion of skin ; aometimes separate analyses have to be made of different parts of a bar of these amps.
Percentage of water.—About 2 grm. of the soap is expoaed in a wide-mouthed flask of about 100 cc. capacity, to a temperature of 149° (300° F.) in an air- or oil-bath for one hour, and the loss in weight is noted. The flask should be weighed as soon as it is cool, and, where great accuracy is
required, should be cooled under a bell-glass in presence of a strong oil of vitriol, as anhydrous soap is very hygroscopic. The operation may be shortened by one-half if a few drops of alcohol be added as soon as the soap has melted ; the addition of a known weight of fine dry sand preventa the aoap from swelling up too much. No well-made soap should turn brown or discoloured at this temperature.
Percentage of Soda.—A burette (Fig. 1237, p. 1768) is provided, divided into fifths of a cuhic centimetre, and a standard aolution of acid, such as is directed in works ou alkalimetry ; either sul phuric or oxalic acida may be used. To determine the total percentage of soda present, dissolve 5 grm. of the soap in boiling water, aud add to it the standard acid solution, stirring and boiling the whole time, until a permanent froth is no longer viaible ; from the number of cc. of acid used, the amount of soda is readily calculated. To determine the soda uncombiued with fat, dissolve 10 or 20 grm. in water, add enough sodium chloride to precipitate the soap, remove the liquor, re-dis solve the soap in fresh water, repeat the operation, mix both brine solutions together, and estimate the soda therein by standard acid, using litmus to determine when enough has been added. The second result subtracted from the first gives the percentage of soda combined with fatty acids.
Percentage and Examination of Fatty Acids.—A knovvn weight of the soap (10 or 20 grm., if only the percentage is required, 50 or NO grm. if the nature of the fat is to be ascertained) is dissolved in hot water. If any portion refuses to dissolve, as will be the case if ateatite, clay, or starch have been mixed with the &lap, the solution must be filtered, either in a hot closet, or through a funnel surrounded by hot water ; if the filter be previously weighed, the insoluble portion can be weighed upon it after being washed and dried at or above 100° (212° F.); to the clear amp solution, an excess of sulphuric or hydrochloric acid is added, and the whole is gently boiled until the fatty acids are clear and transparent, and all cloth have disappeared. If there is reason to believe that the fatty acids will be fluid, or even soft, and greasy, at the ordinary temperature, and a fat percentage only ia desired, a weighed quantity of white wax or atearic acid, previously deprived of water [see Oils, p. 1462, (1) a.], should be added at this atage. When the cake of fatty acida ia cold, tLe liquid beneath should be removed, and the cake remelted over fresh hot water, to remove all traces of salts and acids. When cold, it may be partially dried with blotting-paper, if it is solid enough not to give up any oleic acid to that absorbent ; it should then be all carefully transferred to a tared capsule, heated to at least 127° (260° F.) to expel the last traces of meehanically mixed water, and then weighed, the weight of wax or stearic acid added being of course deducted. Every 100 parts of fatty acids so obtained, represent 105-106 parts of pure neutral fat used to make the soap. In this condition, the fatty acids are hydrates, and from every 100 parts, 3.5 parts must be deducted in stating the analytical results for water chemically combined with them, because anhydrous soap [dried at 149° (300° F.)] does not contain these elementa of water.