THE CHEMISTRY OF CLEANING. 



105 



It is the x>resence of this oily secretion which holds the dirt 

 glued to the skin, and being also rubbed off on the inside of the 

 wristbands and collars of our shirts, causes these portions of our 

 linen to become the most soiled. We may look upon this form 

 of dirt, therefore, as being glued on to the surface by oleaginous 

 materials, which being insoluble in water resist any mere rinsing ; 

 and the most important function of our cleansing materials is to 

 provide a solvent which shall be able to loosen the oil, and so 

 allow of the removal of dirt from the skin. The skin, however, 

 is not the only source of oily matter, and in all fibers of animal 

 origin more or less fat is to be found, which, although not in suf- 

 ficient quantity to play any very important part in the fixation of 

 dirt, still adds its iota to the general result. 



We notice, moreover, that the air of a big town has a far 

 greater dirtying effect than country air, this being partly due to 

 the fact that the number of solid particles per cubic foot of atmos- 

 phere are greatly increased, but chiefly because country air does 

 not contain certain products of incomplete combustion, which are 

 to be found in all large towns. In London we annually consume 

 some six million tons of bituminous coals, and if we examine the 

 smoke which escapes up our chimney during the imperfect com- 

 bustion which the coals undergo in our fire grates, we find that 

 not only will that smoke contain small particles of unconsumed 

 carbon in the form of blacks or soot, but also a considerable 

 quantity of the vapor of condensible hydrocarbon oils, which, 

 depositing on the surface of the solid particles of floating dirt, 

 gives them an enhanced power of clinging to any surface with 

 which they come in contact. 



Hydrocarbon oils of this character are not as a rule affected 

 by the solvents which we utilize for loosening the dirt which is 

 held to our skin by animal grease; but there is no doubt that 

 the dirtying influence of town air is greatly increased by their 

 presence. 



If we take any grease of vegetable or animal origin, we find 

 that it can be dissolved in liquids containing free alkalies, this 

 term being applied to the compounds formed by water with the 

 soluble metallic oxides, which, when dissolved in water, give solu- 

 tions having a soaplike taste, affecting the color of vegetable ex- 

 tracts, such as that obtained by the red cabbage, and possessing 

 the power of neutralizing the acidulous properties of the com- 

 pounds we call acids. 



If we take two metals discovered by Sir Humphry Davy in 

 1807 potassium and sodium and expose them to dry, pure air, 

 they rapidly become converted into a white powder by absorbing 

 oxygen from the atmosphere, and form compounds which we term 

 respectively oxide of sodium and oxide of potassium. These 



