46 
PROFESSOR TYNDALL ON THE OPTICAL DEPORTMENT OF THE 
passing through the liquid as through a vacuum. A comparison of the light with that 
scattered by such mastic particles as those above referred to, proved the suspended particles 
of the ice- water to be far smaller than those of the mastic. No microscope, therefore, 
could come near them*. Such water, however, was proved by Dr. Sanderson to be 
as infectious as the water from any ordinary tap. 
Infinitesimal as these particles are, however, they may be separated by mechanical 
means from the liquid in which they are held in suspension. Filters of porous earthen- 
ware, such as the porous cells of Bunsen’s battery, have been turned to important account 
in the researches of Dr. Zahn, Professor Klebs, and Dr. Burdon Sanderson. In various 
instances it has been proved that, as regards the infection of living animals, the porous 
earthenware intercepts the contagium. For the living animal, organic infusions or 
Pasteur’s solution may be substituted. Not only are ice-water, distilled water, and 
tap-water thus deprived of their powers of infection, but, by plunging the porous cell 
into an infusion swarming with Bacterial life, exhausting the cell, and permitting the 
liquid to be slowly driven through it by atmospheric pressure, the filtrate is not only 
deprived of its Bacteria , but also of those ultra-microscopic particles which appear 
to be as potent for infection as the Bacteria themselves. The precipitated mastic 
particles before described, which pass unimpeded through an indefinite number of 
paper filters, are wholly intercepted by the porous cell. 
These germinal particles abound in every pool, stream, and river. All parts of the 
moist earth are crowded with them. Every wetted surface which has been dried by 
the sun or air contains upon it the particles which the unevaporated liquid held in 
suspension. From such surfaces they are detached and wafted away, their universal 
prevalence in the atmosphere being thus accounted for. Doubtless they sometimes 
attach themselves to the coarser particles, organic and inorganic, which are left behind 
along with them ; but they need no such rafts to carry them through the air, being 
themselves endowed with a power of flotation commensurate with their extreme smallness 
and the specific lightness of the matter of which they are composed. 
I by no means affirm that the developed Bacterium , which requires for its maintenance 
nutriment beyond that which ordinary water can always supply, is never wafted through 
the air. Cases doubtless will arise favourable for the growth and dispersion of 
the full-grown organism. "Whether, after desiccation, it retains the power of repro- 
duction is another question. But it ought, I think, to be steadily borne in mind 
that the Bacteria and the atmospheric matter from which they are developed are, 
in general, different things. I have carefully sought for atmospheric Bacteria , but have 
never found them. They have never, to my knowledge, been found by others ; and that 
they arise from matter which has not yet assumed the Bacterial form is, as just shown, 
capable of demonstration. An organic infusion, boiled and shielded from atmospheric 
particles, will remain clear for an indefinite period, while a fragment of glass which 
* I have endeavoured to convey some notion of the smallness of these scattering particles in ‘ Fragments of 
Science,’ 1876, pp. 441-443. See note on Hr. Dallinger’s observations at the end of this Memoir. 
