The cleaning of the dishes, silver, cutlery, and linen introduces a great variety of chemical problems. The subject of the chemistry of cleaning may well include with the daily task of dishwashing, the equally important ones of house cleaning and laundry work.

The various processes of housework give rise to many volatile substances, such as the vapor of water or fat. If not carried out of the house in their vaporous state these cool and settle upon all exposed surfaces, whether walls, furniture, or fabrics. This thin film entangles and holds the dust, clouding and soiling with a layer more or less visible everything within the house. The fires and lights give out smoky deposits of incomplete combustion. The dishes are soiled with waste from all kinds of foods - starch, grease, albumin, milk, gums, or gelatines and the juices of fruits.

Dust alone might be removed from most surfaces with a damp or even with a dry cloth, or from fabrics by vigorous shaking or brushing; but usually the greasy or sugary deposits must first be broken up and the dust thus set freer This must be accomplished without harm to the material which is dirty.

Cleaning, then, involves two processes: (I) the greasy or gummy film must be broken up, that the entangled dust and dirt may be set free; (2) the dust must be removed by mechanical means.

We will have occasion to use alkalis for cleaning and acids for removing stains and it will be well to consider what is meant by the terms, acid, alkali, and salt.

An acid is a substance with an acid or sour taste and having the property of changing certain vegetable colors. A substance much used in testing for acids is litmus, a kind of fungus, giving a blue solution in water. Paper soaked in litmus solution and dried is known as test paper or litmus paper. It can be bought at any druggist's. This paper is turned red by the presence of any acid, even in the most minute quantity. An acid will cause effervescence with a carbonate like cooking soda or washing soda.

An alkali is a substance often having a soapy taste, a slippery feeling if strong, and the property of turning red litmus, blue.

Alkalies will neutralize the effects of acids. If an acid be added very carefully to an alkaline solution, there comes a point where the mixture will change the color of litmus in neither direction. The solution is neither acid nor alkaline, and is said to be neutral. If we make a weak solution of the acid sold at the drug stores as muriatic acid, and add to this very carefully a weak solution of caustic soda, until the solution is neutral, we shall find that the neutral solution will taste like table salt. In fact, we have made common salt in this way.

A chemical salt is a substance obtained by neutralizing an acid with an alkali or otherwise - a substance that is usually neutral and will turn the color of neither red nor blue litmus paper.

All acids contain the element hydrogen, which can often be driven out and replaced by a metal placed in the acid. If we drop a bit of zinc into some muriatic acid, tiny bubbles of hydrogen begin to escape. The zinc joins the remainder of the acid, making a new substance. This new substance is the metallic salt, called muriate (or chloride) of zinc. Muriatic acid is also called hydrochloric acid. Thus a salt results from neutralizing an acid with a metal. If oxide of zinc, a white powder, has been used in place of the metal, the same salt, chloride of zinc, would have been made; but no hydrogen gas would have come off, for the hydrogen of the acid would unite with oxygen of the oxide and form water.

Grease or fats, called oils when liquid at ordinary temperature, are chemical compounds made of carbon, oxygen, and hydrogen combined in many different ways, but all contain an ingredient of an acid nature known to the chemist as a fatty acid. The fatty acid base is combined with glycerine in the common fats.

Strong alkaline substances will break up fats into their parts and combine with the fatty acid, thus making soap.

The elements which form strong alkalis are the "alkali metals." The common elements of this group are sodium and potassium. There is also ammonium which is not an element, but a combination of nitrogen and hydrogen; it acts, however, like an alkali metal.

When an element unites with water in a certain way it is called a hydrate or hydroxide. The hydrate of ammonium - aqua ammonia or ammonia - is known as the "Volatile alkali" because it evaporates so easily. It is valuable for use in all cleansing operations - in the kitchen, the laundry, the bath, in the washing of delicate fabrics, and in other cases where its property of evaporation, without leaving any residue to attack the fabric or to absorb anything from the air, is invaluable.

The hydrates of potassium and sodium are called caustic potash and caustic soda, respectively, or the caustic alkalis or "lyes" because they "burn" animal tissues. These combine readily with fats to form compounds which we call soaps.

Most of the fats are soluble in turpentine, ether, chloroform, naphtha, or kerosene, and somewhat in alcohol. That is, the fats are dissolved unchanged, just as salt is taken up by water. These form solvents for greases more or less valuable according to conditions.

If the housekeeper's problem were the simple one of removing the grease alone, she would solve it by the free use of one of the solvents or by some of the strong alkalis. This is what the painter does when he is called to repaint or to refinish; but the housewife wishes to preserve the finish or the fabric while she removes the dirt. She must, then, choose those materials which will dissolve or unite with the grease without injury to the article cleaned.

Soap is by all odds the safest and most useful cleaning agent. It is made from most of the common animal and vegetable fats and oils, as tallow, suet, lard, cotton seed oil and cocoanut oil, chemically combined with caustic soda or caustic potash. Castile soap is supposed to be made from olive oil. Rosin soap forms a part of all common yellow soap. It lessens the cost and makes a good soap for rough work. Silicate of soda is sometimes added to cheap soaps. It has some cleansing action, but must be regarded as an adulterant.