TYNDALL ON HAZE AND DUST. 



691 



Air was then passed through a tube which 

 contained a roll of platinum gauze ; and it was 

 found that, when the platinum was cold, the 

 dust-particles all passed through with the air, 

 but, when it was made red-hot, the dust-par- 

 ticles were all consumed. In this case, too, 

 when the air was forced quickly through, a 

 fine blue cloud of smoke appeared, just as in 

 the experiment with the spirit-lamp. An at- 

 tempt was then made to burn the dust-par- 

 ticles by the concentrated rays of a convergent 

 mirror, but it failed ; the particles flitted too 

 quickly through the focus of the burning ray 

 to be consumed by it. 



The next experiment was to put the flame 

 of a spirit-lamp in the ray of light which was 

 revealing the floating dust. At once the flame 

 was seen to be surrounded by wreaths of dark- 

 ness, resembling intensely-black smoke. On 

 lowering the flame beneath the beam of light, 

 the same dark masses were seen wreathing 

 upward. "They were, at times," said Dr. 

 Tyndall, "blacker than the blackest smoke 

 that I have seen issuing from the funnel of a 

 steamer, and their resemblance to smoke was 

 so perfect as to lead the most practiced ob- 

 server to conclude that the apparently pure 

 flame of the alcohol required but a beam of 

 sufficient intensity to reveal its clouds of lib- 

 erated carbon." But, when a red-hot poker 

 was placed under the beam, the same black 

 wreaths came floating through. A hydrogen 

 flame was next put under it, and the whirling 

 masses of darkness wreathed upward more 

 copiously than ever. The blackness was, 

 therefore, nothing but air from which all dust- 

 particles had been burned out, and which, 

 consequently, contained nothing to catch the 

 light and reflect it to the eye, as the dust-par- 

 ticles do. 



Here, however, a difficulty came in. The 

 same effect was produced by a copper ball not 

 hot enough to burn the dust, and by a flask 

 filled with hot water. In this case it was 

 found that the air was rarefied with the 

 warmth, and, as the dust-particles were not 

 heated to the same extent, it dropped them 

 and floated upward without them. Other 

 gases, even common coal-gas, carefully pre- 

 pared so as to exclude the dust-particles, have 

 the same black appearance when they cross a 

 ray, which the dust-laden air renders visible, 

 and if coal-gas or hydrogen be let into the top 

 part of a glass shade, which has been placed 

 in a sunbeam or a ray of the electric light, 

 the line between the dust-laden air and the gas 

 is rendered visible where the air is, the shade 

 will seem full of the illuminated particles; 

 where the gas is, it will appear absolutely 

 empty. " The air of London rooms is filled 

 with this organic dust, nor is the country 

 air free from its pollution. It only needs a 

 sufficiently powerful beam to make the air 

 appear as a semi-solid rather than a gas." 



Nobody could, in the first instance, without 

 repugnance, place the mouth at the illuminated 



focus of the electric beam and inhale the dirt 

 revealed there. Yet we are inhaling it every 

 moment, and the wonder is, that so small a 

 portion of it should be injurious to health. 



What is the portion of this ever-present and 

 all-pervading dust which is injurious to life? 

 Now, it was long believed that epidemic dis- 

 eases were propagated by malaria, which con- 

 sisted of organic matter in a state of motor- 

 decay ; that, when such matter was taken into 

 the body through the lungs or the skin, it had 

 the power of spreading in it a similar decay 

 yeast was a case in point. "Why should not a 

 bit of malarious matter operate in the body as 

 a little leaven, leavening the whole lump? 

 But, in 1836, Cagniard de la Tour discovered 

 the yeast-plant, which, when placed in a 

 proper medium, grows and spreads, and pro- 

 duces what we call fermentation. In the next 

 year Schwann, of Berlin, discovered the plant 

 independently. He also proved, that when a 

 decoction of meat is effectually excluded from 

 common air, and supplied solely with air 

 which has been raised to high temperature, it 

 never putrefies. Putrefaction, therefore, he 

 said, came from the air, and could be destroyed 

 by a sufficiently high temperature. Helmholtz 

 and Ure repeated and confirmed his experi- 

 ments ; but the high authority of Gay-Lussac, 

 who ascribed putrefaction to oxygen, drove 

 chemists back on the old notion. That notion 

 was finally exploded by Pasteur, who proved 

 that ferments are organized beings which find 

 in what we call ferments their necessary food. 



Side by side with these discoveries grew up 

 the germ-theory of epidemic disease. Kircher 

 expressed the idea, and Linna3us favored it, 

 that epidemic diseases are due to germs which, 

 floating in the atmosphere, enter the body 

 and produce disease by the development of 

 parasitic life. Sir Henry Holland has favored 

 this theory, which derives its strength from 

 the perfect parallelism between the phenomena 

 of contagious disease and those of life. As an 

 acorn planted in the soil gives birth to an oak 

 which produces a whole crop of acorns, each 

 of which has power to reproduce its parent 

 tree, and thus, from a single seed, a whole 

 forest may spring, so a germ of disease, planted 

 in the human body, grows and shakes abroad 

 new germs, which, meeting in other human 

 bodies with their proper food and tempera- 

 ture, finally take possession of whole popula- 

 tions. Thus, Asiatic cholera, beginning in a 

 small way in the delta of the Ganges, spread 

 itself, in seventeen years, over nearly the 

 whole habitable world. 



An infinitesimal speck of small-pox .virus 

 will develop a crop of pustules, each charged 

 with the original poison. The reappearance 

 of this scourge, as in the case of the Dread- 

 nought, at Greenwich, so ably reported on by 

 Dr. Budd and Mr. Busk, is explained by the 

 theory which ascribes it to the lingering of 

 germs about the infected place. Surgeons 

 have long known the danger of admitting air 



