Abstract of a Lecture delivered at the Royal Institution. From the "British Medical Journal," January 29th, 1870.

Professor Tyndall, in making some experiments on vapours, wished for a current of air quite free from the particles of dust always seen in a beam of sunlight. He tried various means for this purpose; one consisted in passing air through a tube filled with sulphuric acid; another, in passing the air through a tube filled with solution of potash. In each case, particles, capable of refracting light, and rendering themselves visible, were still present. In October, 1868, he hit on the plan of allowing the air to pass over the flame of a spirit-lamp. 'The floating matter no longer appeared, having been burnt up by the flame. It was, therefore, organic matter. If the air were sent too rapidly through the flame, a fine blue cloud was noticed. This was the smoke of the organic particles.' The Professor was not prepared for the discovery that the dust of our air was organic. He had always considered it inorganic and non-combustible. M. Valentin now furnished him with a small gas furnace with a platinum tube which could be heated to redness. Air was passed through this tube when cold, and then when hot. When combustion was perfect, no particles could be detected.

Further experiments led to still more interesting results. A beam of light was made to illumine the dust of the laboratory, and the flame of the spirit-lamp allowed to play on it. Wreaths of darkness were at once seen to mingle with the flame, just like intensely black smoke. 'When the flame was placed below the beam of light, the same dark masses steamed upwards.' They were at times blacker than the blackest smoke. A red-hot poker placed under the beam produced the same dark wreaths. A large hydrogen flame led to the same result. Smoke was therefore out of the question. What then, was the blackness? Simply that of stellar space, resulting from the absence, from the track of the beam, of all matter capable of scattering its light.

The Professor then remarked: 'Nobody could, without repugnance in the first instance, place the mouth at the illuminated focus of the electric beam and inhale the dirt revealed there. Nor is the disgust abolished by the reflection that, although we do not see the nastiness, we are churning it in our lungs every hour and minute of our lives.' The wonder is, that so small a portion of this dirt should appear to be really deadly to man. What is this portion?

The lecturer then alluded to the notion, at one time prevalent, that malarious diseases were due to organic matter in a state of decay (fermentation). It was then shown that fermentation really depended on the growth of the yeast-plant. Further, Schwann, in 1837, showed that meat, in contact with air which had been heated, did not putrefy; and he affirmed that putrefaction was caused by something derived from the air, which could be destroyed by high temperature. The germ-theory of epidemic disease soon followed, and found an energetic supporter in Sir Henry Holland, the present President of the Royal Institution. The spread of cholera and that of small-pox were adduced as instances in support of the germ theory.

Professor Tyndall alluded to the difficulty that must be experienced in freeing surgical instruments (a canula, for instance) from the means of carrying contagion, in the presence of an atmosphere such as ours, unless a high temperature were employed, and this is not done. Thus, notwithstanding all the surgeon's care, inflammation often sets in after the use of such an instrument. When an abscess has been tapped, the pus, which was at first sweet, becomes foetid and swarms with vibrios. Professor Lister's views were quoted.

The Professor then said the dust could not be blown away with a pair of bellows; but, if the muzzle of the bellows were stuffed with cotton wool, it was found that the air which escaped was free from particles. Schroeder used cotton-wool as a filter in his experiments on spontaneous generation; and subsequently it was used in those of Pasteur. Since 1868 Dr. Tyndall has used it himself.

The most interesting and important illustration of such a filtering process is furnished by the human breath. After inspiring a quantity of common air, a long expiration is made through a glass tube across the electric beam. At first the luminous track is uninterrupted. The breath impresses on the floating matter a transverse motion, but the dust from the lungs makes good the particles displaced. After a time, however, an obscure disc appears upon the beam, and, at the end of expiration, the beam is, as it were, pierced by an intensely black hole, in which no particles whatever can be discerned. The air, in fact, has lodged its dirt in the lungs. A handful of cotton-wool placed over the nose and mouth during inspiration makes the dark hole in the beam of light appear from the beginning of expiration. A silk handkerchief answers nearly as well, but the filtration is not nearly so perfect as with cotton-wool.

"In conclusion, the use of cotton-wool respirators was strongly advocated."