After giving the details and results of a series of experiments in which he attempted to extract the dust from the air of the Royal Institute by passing it through a tube containing fragments of glass wetted with concentrated sulphuric acid, and thence through a second tube containing fragments of marble wetted with a strong solution of caustic potash, which experiments were attended with perfect failure, the Professor continues, "I tried to intercept this floating matter in various ways; and on the day just mentioned, prior to sending the air through the drying apparatus, I carefully permitted it to pass over the tip of a spirit-lamp flame. The floating matter no longer appeared, having been burnt up by the flame. It was, therefore, of _organic origin_. I was by no means prepared for this result; for I had thought that the dust of our air was, in great part, inorganic and non-combustile." In a foot note he says, "according to an analysis kindly furnished me by Dr. Percy, the dust collected _from the walls_ of the British Museum contains fully fifty per cent of inorganic matter. I have every confidence in the results of this distinguished chemist; they show that the _floating_ dust of our rooms is, as it were, winnowed from the heavier matter." Again he says: "the air of our London rooms is loaded with this organic dust, nor is the country air free from its presence. However ordinary daylight may permit it to disguise itself, a sufficiently powerful beam causes dust suspended in air to appear almost as a semi-solid. Nobody could, in the first instance, without repugnance, place the mouth at the illuminated focus of the electric beam and inhale the thickly-massed dust revealed there. Nor is the repugnance abolished by the reflection that, although we do not see the floating particles, we are taking them into our lungs every hour and minute of our lives." "The notion was expressed by Kircher and favored by Linnaeus, that epidemic diseases are due to germs which float in the atmosphere, enter the body, and produce disturbance by the development within the body of parasitic life. While it was struggling against great odds, this theory found an expounder and a defender in the President of this institution. At a time when most of his medical brethren considered it a wild dream, Sir Henry Holland contended that some form of the germ-theory was probably true." Professor Tyndall proposes means by the application of which air loaded with noxious particles may be freed from them before entering the air passages. The following embodies his suggestions on this point: COTTON-WOOL RESPIRATOR.