30. DUST IN THE AIR.--Attention has lately been directed to the dust, or haze, that marks the ray of sunshine across a shaded room. Just as, many years ago, it was discovered that myriads of animalcula infested much of the water we drank, so now the microscope reveals "the gay motes that dance along a sunbeam" to be, in part, composed of multitudes of animal and vegetable forms of a very low grade, the germs of fermentation and putrefaction, and the probable sources of disease. [Sidenote: 31. The best air filter? The remarks of Prof. Tyndall?] 31. It is found that the best filter by which to separate this floating dust from the air is cotton wool, although a handkerchief will imperfectly answer the same purpose. In a lecture on this subject by Prof. Tyndall, he remarks that, "by breathing through a cotton wool respirator, the noxious air of the sick room is restored to practical purity. Thus filtered, attendants may breathe the air unharmed. {138} In all probability, the protection of the lungs will be the protection of the whole system. For it is exceedingly probable that the germs which lodge in the air-passages are those which sow epidemic disease in the body. If this be so, then disease can certainly be warded off by filters of cotton wool. By this means, so far as the germs are concerned, the air of the highest Alps may be brought into the chamber of the invalid." [Sidenote: 32. Carbonic acid in volcanic regions? In Java? At Lake Avernus? In mines?] 32. CARBONIC ACID IN THE AIR.--We have already spoken of this gas as an exhalation from the lungs, and a source of impurity; but it exists naturally in the atmosphere in the proportion of one half part per thousand. In volcanic regions it is poured forth in enormous quantities from fissures in the earth's surface. Being heavier than air, it sometimes settles into caves and depressions in the surface. It is stated that in the island of Java, there is a place called the "Valley of Poison," where the ground is covered with the bones of birds, tigers, and other wild animals, which were suffocated by carbonic acid while passing. The Lake Avernus, the fabled entrance to the infernal regions, was, as its name implies, bird-less, because the birds, while flying over it, were poisoned by the gas and fell dead into its waters. In mines, carbonic acid forms the dreaded _choke-damp_, while carburetted hydrogen is the _fire-damp_. [Sidenote: 33. In the open air? Amount of carbonic acid exhaled by a man? A gas-burner? A room fire? From furnaces?]