io6 THE POPULAR SCIENCE MONTHLY. 



oxides, when dissolved in water, enter into combination with a 

 portion of it, producing sodic hydrate and potassic hydrate, two 

 substances which have pre-eminently the properties which we 

 term alkaline, and which exert a strong solvent action upon all 

 forms of animal and vegetable grease. These solutions exercise 

 a wonderful power of cleansing upon the grease-bound particles 

 of dirt which veil our skin, but so strong is their solvent power 

 upon animal membrane, that not only do they dissolve fatty mat- 

 ter, but also the cuticle itself, so that they are manifestly unfitted 

 for removing dirt from a tender skin, and we are forced to look 

 further afield for a grease solvent. 



If instead of dissolving our sodic and potassic oxide in water 

 we had left them exposed to ordinary air, we should have found 

 that they gradually attracted from the atmosphere a gas called 

 carbon dioxide, which exists in all air to the extent of four parts 

 in ten thousand, and that by combining with this gas they be- 

 came converted into sodic and potassic carbonates, bodies which 

 we call salts, and which, although not so violent in their action 

 upon the skin, will retain to a certain extent their solvent action 

 on fatty matters. 



The carbonates of sodium and potassium are found in the 

 ashes of many vegetable and animal substances, and in the earli- 

 est records which have been discovered we find mention of the 

 cleansing power of wood ashes, the ashes of certain marine 

 plants, seaweed, and " natron," which is an alkaline efflorescence 

 from some kinds of soil ; nor has the use of ashes for this pur- 

 pose entirely died out at the present time. 



As early as a. d. 69, however, we find that the elder Pliny 

 mentions another form of cleansing material made from tallow 

 and ashes, the components piost recommended being goat's suet 

 and the ash of beech wood ; while the ruins of Pompeii were found 

 to contain a fairly perfect soap factory. 



Although soap and Christianity date from the same period, it 

 was only at the commencement of this century that the classical 

 researches of Chevreul on the constitution of fats gave the key 

 to the reactions taking place during its formation, while even at 

 the present time we probably only know a true explanation of 

 part of the actions which lead to its cleansing effect upon the 

 skin. 



If we take sulphuric acid diluted with water, we find that it 

 has certain well-marked characteristics which leave no room for 

 doubting its acidulous nature ; and if we pour a few drops of it 

 into the violet-colored solution obtained by boiling sliced red cab- 

 bage in water, the violet solution at once becomes bright red. On 

 repeating this experiment with the violet cabbage solution and 

 a few drops of sodic hydrate solution, we obtain a vivid green 



