Moist air is very objectionable to the housewife for another cause - it rusts her bright stoves and fire-irons. The oxygen in the air combines with the surface of the metal and oxidizes - that is, rusts - the iron. The only way to prevent iron from rusting in moist air is to grease it - grease preventing the humidity of the air coming in contact with the surface of the iron.

The oxygen of dry air will not rust iron; moisture (or a very great degree of heat) is necessary to bring into action the affinity of oxygen for steel. Copper and zinc also are tarnished by oxidation. The oxygen from moist air combines with the surface of these metals, and tarnishes them; they do not rust as steel does. Lead is rendered dull by moist air in the same way, and silver oxidizes and tarnishes. Platinum is the only metal which never oxidizes in moist air. Gold also is little affected by the atmosphere if it - the gold - be quite pure.

We must now speak of water as a portion of our food, and a very important portion it is.

It is contained more or less in everything we eat; when the food is of a dry nature, as wheat, peas, oatmeal, maize, etc., more drink is required.

Spring-water is more agreeable to drink than soft water, because it contains carbonic acid which gives it the life and sparkle we all prefer, and singularly enough the acid so deadly to inhale is conducive to health when drunk in water, acting as a refrigerant and allaying irritability of stomach.

The carbonic acid is produced in water by the presence of bicarbonate of lime in it. It escapes very quickly into the air, leaving the water flat and stale, therefore it is best drunk fresh from the pump, and should not be let stand long uncovered before using.

The quality of the water of springs depends a great deal on the soil through which they pass; from granite and slate formations it is very pure; sandstone is inferior in this respect, while lime and magnesia render the water, as we have seen, very hard, and if found in large quantities, make it injurious to health. Hard water is found most commonly to contain chalk - i.e., carbonate of lime.

If the presence of this carbonate is suspected - and it will lurk in clear bright sparkling filtered water - it may be tested by boiling, when the chalk will at once be thrown as a "furr "* or coating on the inside of the vessel in which it has been boiled.

If the lime in the water, however, is in the condition of the sulphate, - that is, gypsum, no amount of boiling will precipitate it. The only thing then to be done is to put carbonate of soda into the water in sufficient quantity to convert the sulphate of lime into the carbonate. If the water be then boiled the chalk will be deposited as furr, and sulphate of soda will be left in the water. Hard water may be made softer by exposure to the air in a wide vessel - as a flat pan. The carbonic acid in it will escape into the air, and the mineral salts which cause its hardness will subside. It is lime and magnesia in various forms which cause "hardness" in water.

* This furr may be removed from kettles by boiling in them a little sal ammoniac. The hydrochloric acid unites with the lime of the furr, and the carbonic goes to the ammonia. Both of these new compounds dissolve, and can be very easily washed away.

Water contaminated by lead cisterns or pipes has proved very injurious. A ninth part of a grain of lead per gallon will affect the health of a whole family. But happily this injurious substance may be easily removed by filtering the water through sand.

Water may be purified in many ways. Pouring it from a height into a flat pan, so as to allow the air to mix with it will improve it. Stirring it about with freshly-made animal charcoal purifies it. You can purify a gallon of water with twenty drops of sulphuric acid. "An ounce of powdered alum dissolved and stirred into a hogshead of putrid water will precipitate the foul matter in a few hours, and render it pure again".

Chips of oak wood put into water will purify it. The tannic acid in the oak effects this change. The worst impurity in water proceeds from sewage, which is apt to get into rivers and old wells. Nitrogenous products when they become mixed with a large quantity of river water, absorb oxygen and are slowly converted into inorganic matter, the nitrogen they contain being converted into nitrous and nitric acids. These acids unite with lime, etc., and form nitrites and nitrates, while the carbon of which they were composed becomes carbonic acid. The hydrogen in them is formed into water. If these nitrites or nitrates of lime and magnesia are found in any water used for drinking purposes it should be avoided.

Generally water had better be filtered, as it might look both bright and sparkling, and be even pleasant to the taste, and yet contain many impurities. Filters are very cheap - a pretty glass one resembling an hour-glass in form, can be bought for 7s. 6d. at the Crystal Palace; or any poor man may make himself one in accordance with the directions given at the South Kensington Museum, which we transcribe for our readers.