nitrogen, and two accessory elements which vary extremely in amount, but are practically never absent, namely, carbonic acid and water. Without either of the first two, air could not exist, and without the last two, air is scarcely found in nature. Their combination moreover is not a chemical union but a simple mechanical mixture. But besides these constituents the air contains an immense amount of life, and small particles derived from the whole creation. In the air may be found animalcules, spores, seeds, cells of all kinds, eggs of insects, fungi and elements of contagion, besides formless dust, and sandy and other particles of local origin. For example, no one can travel in a railway carriage without being surrounded by dust, a large portion of which may be attracted by a magnet, consisting as it does in a great measure of minute particles of iron derived from the rails. The purest air has some dust in it. There probably never fell a beam of light from the sun since the world was made which would not have shown countless numbers of solid particles. Roughly speaking, 100 measures of air, if pure, should contain 78.98 parts of nitrogen, 20.99 of oxygen, and .03 of carbonic acid. Without oxygen a candle will not burn, and animals can not live; but for the purposes of animal life this gas requires to be diluted, and this is effected in the atmosphere by a large admixture of nitrogen. In fact, nitrogen seems to act in the animal economy purely as a diluent or vehicle for the administration of oxygen. Carbonic acid, as far as we know, is not essential to the animal kingdom. To man it is simply a superfluous ingredient, but harmless when in small quantity; to the vegetable world, on the contrary, it is a food which together with water often suffices to support the entire life of a plant. When, however, from any cause the quantity of carbonic acid is much increased, it becomes highly poisonous to man. When the amount reaches 10 or even 5 volumes per cent. it produces fatal results, and even 2 per cent. occasions in most persons severe headache. The balance between carbonic acid and oxygen in the atmosphere, continually disturbed in one direction by the animal kingdom, is constantly maintained by the vegetable kingdom; for while the former consumes oxygen and gives off carbonic acid, the latter for the most part performs the opposite function. Owing to certain local conditions, however, which we shall presently consider, the quantity of oxygen in the atmosphere sometimes falls below the normal amount to the extent of over 3 per cent., while the carbonic acid proportionately increases. In order to estimate the importance of what might otherwise appear trifling differences in the composition of the air we breathe, we must remember that we take into our lungs from 1,000 to 2,000 gallons of air daily. Now the presence of only a few grains of impurity in a gallon of water would render it unfit for drinking purposes,