I tried to intercept this floating matter in various ways; and on October 5, 1868, prior to sending the air through the drying apparatus, it was carefully permitted 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 _organic matter_. I was by no means prepared for this result; having previously thought that the dust of our air was, in great part, inorganic and non-combustible. [Footnote: According to an analysis kindly furnished to me by Dr. Percy, the dust collected _from the walls_ of the British Museum contains fully 50 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. As bearing directly upon this point I may quote the following passage from Pasteur: 'Mais ici se présente une remarque: la poussière que Pon trouve à la surface de tous les corps est soumise constamment à des courants d'air, qui doivent soulever des particules les plus légères, au nombre desquelles se trouvent, sans doute, de préférence les corpuscules organisés, oeufs ou spores, moins lourds généralement que les particules minérales.'] I had constructed a small gas-furnace, now much employed by chemists, containing a platinum tube, which could be heated to vivid redness. [Footnote: Pasteur was, I believe, the first to employ such a tube.] The tube contained a roll of platinum gauze, which, while it permitted the air to pass through it, ensured the practical contact of the dust with the incandescent metal. The air of the laboratory was permitted to enter the experimental tube, sometimes through the cold, and sometimes through the heated, tube of platinum. In the first column of the following fragment of a long table the quantity of air operated on is expressed by the depression of the mercury gauge of the air-pump. In the second column the condition of the platinum tube is mentioned, and in the third the state of the air in the experimental tube. Quantity of air State of platinum tube State of experimental tube 15 inches Cold Full of particles. 30 inches Red-hot Optically empty. The phrase 'optically empty' shows that when the conditions of perfect combustion were present, the floating matter totally disappeared. *****