HURSDAY, JANUARY 
an 
27, 1870 
DUST AND DISEASE 
ROFESSOR TYNDALLS lecture last Fridaynight at 
the Royal Institution, which we give zz exfenso in 
another column, has excited unusual interest, not only 
on account of the Professors beautiful demonstration of 
the presence of organic dust in our London atmosphere, 
but from the manner in which he has sought to connect 
this presence with certain theories of disease held by 
part of his audience. 
The revelations made by an ordinary sunbeam passing 
through a hole in the shutter are familiar enough; and the 
changes produced on the beam by a candle flame ora red- 
hot poker (an experiment used long ago by Dr. Wollaston 
for another purpose) will show in a rough manner the 
nature of the investigations. 
Their practical result may be thus stated: London air 
contains a large amount of organic particles powerfully 
reflecting a light thrown upon them ; and these particles 
cease to reflect light when the air containing them is 
submitted to the action of a high temperature, or when it is 
passed througha strainer or filter. Some of the appearances 
produced by certain modifications of the experiments are 
striking. Such, for example, as the disappearance of 
reflected light when air of a temperature below the incan 
descent point is made to rise through the beam showing 
the dust ; or when certain gases are used instead of 
warmed air. In these cases the reflection of light from 
the dust particles disappears, and blackness takes its 
place. We are not, however, convinced that the Profes- 
sor’s explanation of this striking phenomenon is quite 
tenable. It presupposes a rapidity in the gas and air 
currents greater than can be followed by the more sluggish 
dust particles; so that these are left behind, or thrown to 
one side, and the rarefied air or gas deprived of dust par- 
ticles enters the beam, and becomes invisible from absence 
of reflecting particles. 
One would suppose that, sooner or later, the particles 
must follow the air in which they float, unless the heated 
air or gas become so much lighter than the particles 
that the latter will tend to fall downwards. This point, 
however, is one which admits of further demonstration. 
The microscope, on its side, has not been behindhand 
in the same field, and has told us something more about 
this organic air dust. It is found to vary in character 
according to the objects from which it proceeds, and 
according to the degree of ventilation in an apart- 
ment. A microscopist, with his air analyser, would 
very likely have told Professor Tyndall’s audience 
how they were breathing fragments of each other’s 
clothes, and the scurf skin of each other’s hands 
and faces, besides other matters brought into the 
Institution by the listeners, or wafted in through the 
windows; and if a whiff of sewer air had entered 
the room, living wdriones would probably have been 
among the subjects of the microscopist’s demonstra- 
tion. Chemistry also has been at work in the same 
direction, and by means at her disposal she has been able 
to estimate approximately the amount of organic matter 
in air ; and this application of chemical methods is now in 
common use for determining the state of ventilation 
NA TURE 
A 
327 
in inhabited buildings, as well as the comparative purity 
of the air in town and country. We are glad that Prof. 
Tyndall has enlisted optical analysis in the same 
useful field of inquiry. His lecture, from the perfection 
of his experiments, was well adapted to impress the 
advantages of pure air on the minds of his audience and 
the public at large. 
We cannot help feeling, however, that it would 
have been well if the able lecturer had confined his 
statement strictly to the scientific aspects of his 
subject. The germ theory of disease has nothing in 
common with it, and yet it was referred to as if to show 
that the fact of organic dust existing in the air rendered 
the existence of ‘disease germs,” as they are called, more 
probable than they were before. In scientific subjects we 
cannot accept mere theories for facts. Let the advocates 
of disease germs first prove their existence, and then 
possibly optical and microscopic analysis will throw light 
on their mode of conveyance. 
In imperfect science, as in other imperfect things, the 
first false step may Jead anywhere, as the following extract 
from Dr. Bryden’s singularly interesting report on Indian 
cholera, one of the supposed germ diseases, will show :— 
“* The facts of the first European invasion showed that aérial 
transmission did not account for all the phenomena observed, 
such as the transmission of cholera by fomites, and the occasional 
infection of attendants on the sick. Hence there was initiated 
(a) the doctrine that cholera might be propagated by human in- 
tercommunication, and, as the latest phase of this doctrine. we 
find the confident assertion promulgated as a truth, that cholera 
is always and not occasionally so propagated. To prop 
up this assertion it was necessary to make a second as- 
sumption or theory. And hence arose the doctrine (4) that 
cholera is multiplied in the human economy. But this also 
must have stood alone and unsupported, unless it could be shown 
how and where the multiplication took place. And this led on 
to the starting of the third theory (c), which asserts that cholera 
is multiplied in, and is spread around by, the intestinal evacua- 
tions of those already suffering from the disease. But even this, 
although urged in the most forcible manner, did not meet all diffi- 
culties; and there arose the demand that it should be supplemented 
bya fourth theory. In relation to this demand, the latest theory 
(d) alleges that the evacuations of an individual in whom cholera 
has not become apparent, and never will appear, may be the 
means of spreading cholera around.” 
By endeavouring without observation to attain to know- 
ledge which can only be arrived at by observation, theory 
has, in this instance, walked round in a circle and left 
science outside. In great questions affecting the health 
and life of nations, theories are quite out of place. They 
do no good, cost money, and bar scientific progress. 
Practically, so far as health is concerned, Professor 
Tyndall has given us a scientific account, not only of cer- 
tain optical properties of impure air, but likewise of the 
benefit of several popular practices, such, for example, as 
lighting fires during epidemics to purify the air, the use 
of gauze curtains in malarious districts as a protection 
against fever, covering the mouth with a cloth during 
sleep in fever countries, and the like. He has further 
given us an additional means of estimating the purity or 
impurity of the air we breathe. He has shown that heat 
purifies, more or less, impure air; and that impure air 
can be deprived of its suspended impurity by filtering it, 
as is the case with water. On the real proximate aérial 
cause of disease, if such there be, no new light has been 
yet thrown either by the optician, the microscopist, or 
the chemist, 
