It is the presence 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 sufficient 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 atmosphere 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 combustion 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 solutions having a soaplike taste, affecting the color of vegetable extracts, such as that obtained by the red cabbage, and possessing the power of neutralizing the acidulous properties of the compounds 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