PROFESSOR TYNDALL ON DTJST AND HAZE. 
487 
mirror, but lie was unsuccessful. A spirit-lamp flame was, as lie liad stated, 
capable of causing their combustion, but on placing such a flame beneath a beam 
of light a most curious effect was observed. Wreaths of darkness, resembling 
the blackest smoke, became visible in the luminous ray; and, on substituting 
for the spirit-lamp a large Bunsen burner, a sharply-defined gap was made in 
the beam. Is this darkness smoke? The question was decided when it was 
found that a red-hot iron, or a hydrogen flame, produced the same phenomenon, 
for here there could be no smoke. This darkness, then, said the lecturer, is 
the darkness of the stellar space ; it is owing to the fact that, in the air thus 
deprived by burning of the floating particles, there is nothing to disperse the 
light. It is not, however, always necessary to burn the air. A heated copper 
sphere caused, in common with flame, these wreaths of darkness, but it produced 
them, although to a less degree, after it had cooled below the temperature of 
boiling water. A flask of boiling water was also, to some extent, productive of 
them ; and again, a platinum wire, heated by electricity, gave a similar result 
long before it reached the temperature of ignition. Now, how could this be 
accounted for? In the following manner, said the lecturer:—The air in con¬ 
tact with the heated platinum wire, or copper globe, or flask of boiling water, 
has its temperature raised ; it becomes specifically lighter than the surrounding 
air, and it rises up, and leaves behind it the dust which is not sensibly altered 
in gravity. Various gases, such as hydrogen, prepared so as to exclude the 
floating particles, produce darkness when placed in the track of a ray of light. 
A glass shade, held in the air mouth downwards, allowed the light from the 
electric lamp to pass through it in the form of a visible beam ; but when hydro¬ 
gen or coal-gas was admitted into the shade by means of a tube passing to the 
top, that portion of the luminous ray which was within the glass could no 
longer be seen. 
Professor Tyndall then proceeded to observe that some portion of this atmo¬ 
spheric dust is noxious to human life, and so passed on to the consideration of 
the germ theory of disease. In the treatment of this subject no new facts or 
theories were adduced, and it is probably not saying too much to state that the 
lecture would have been at least as successful had this disquisition on a sub¬ 
ject so essentially medical been omitted. 
This air-dust, said the Professor, cannot be blown away with bellows, and 
when we replace the bellows by the human lungs we simply get a white cloud 
in the ray of light caused by the aqueous vapour in the breath. If, however, 
the breath be previously dried, as, for instance, by exhaling through a heated 
tube, we obtain a somewhat interesting result. On filling the lungs, and slowly 
breathing through the tube into the light ray, we at first get no effect, or prac¬ 
tically none ; but, as the last portions of air are expelled, and pass into the beam, 
we obtain the dark wreaths, indicating that the dust has been removed from this 
part of the air and retained by the lungs. Again, if we fill our lungs with air 
filtered through cotton wool, we find, on exhaling, that the whole of the air 
has been by this medium freed from floating particles. This, said the lecturer, 
accounted for medical men, “ more, perhaps, from instinct than from know¬ 
ledge,” placing their handkerchiefs to their faces on entering an infected atmo¬ 
sphere. That all germs of disease can be intercepted by a filter of cotton wool, 
and that we can thus, as regards germs, bring into our rooms the air of the 
Alps, the lecturer was so strongly convinced that he would willingly test, in his 
own person, the truth of the statement. 
Beyond the means of illustrating to a large audience, there was little new in 
Professor Tyndall’s discourse ; and of that which was novel, it maybe remarked 
that the theory propounded to account for the removal from the air of dust by 
bodies at a temperature below that of ignition is hardly so satisfactory as 
might be desired. The rising of air from a hot body is a continuous process. 
