60 
PROFESSOR TYNDALL ON THE OPTICAL DEPORTMENT OF THE 
Bacteria spring does not exist in ordinary air.” The experiments, however, recorded in 
this memoir constitute an ocular demonstration of the respective parts played by the infu- 
sion and the air. A pinch of fungus-spores, taken between the fingers, sown in a suitable 
medium, and producing their appropriate crop, could not more clearly indicate the origin 
of that crop than experiments with the luminous beam indicate the origin of our 
harvests of Bacteria. Dr. Sanderson is, I doubt not, now well aware that his first statement 
was founded on an error of interpretation. In a lecture delivered at Owens College, Man- 
chester, and published in the ‘British Medical Journal’ for January 16, 1875, he to a 
great extent qualifies and corrects his first inference. He there says that the Bacteria 
“ attach themselves without doubt to these minute particles, which, scarcely visible in 
ordinary light, appear as motes in the sunbeam, or in the beam of an electric lamp.” 
In fact the experiments on which he based his first inference owed their barrenness, not to 
the absence of Bacteria-ge rms from the air, but to the inability or, rather, slowness of his 
mineral solution to develop them. 
With regard to the part played by the visible motes, I may repeat here what has been 
previously stated, namely, that while the coarser particles could hardly exist in their midst 
without loading themselves to some extent with the minute germs of Bacteria , there is 
no reason to think the motes indispensable for the diffusion of the germs. Whether they 
are attached to each other or not, the dryness and the moisture of the air are shared 
equally by both. The germs, moreover, float in the air more readily than the larger 
particles; and they, I doubt not, when properly illuminated, shed forth a portion 
of that changeless light to which reference has been already made, and the perfect 
polarization of which declares the smallness of the masses which scatter it. 
The prevalence of the germinal matter of Bacteria in water has been demonstrated 
by the experiments of Dr. Burdon Sanderson. But the germs in water, it ought to 
be remembered, are in a very different condition, as regards readiness for develop- 
ment, from those in air. In water they are already wetted, and ready, under the 
proper conditions, to pass rapidly into the finished organism. In air they are more or 
less desiccated, and require a period of preparation more or less long to bring them up 
to the starting-point of the water-germs *. The rapidity of development in an infusion 
infected by either a speck of liquid containing Bacteria or a drop of distilled water is 
extraordinary. On the 4th of January I dipped a thread of glass almost as fine as a 
hair into a cloudy turnip-infusion, and introduced the tip only of the glass fibre into a 
large test-tube containing an infusion of red mullet : twelve hours subsequently the per- 
fectly pellucid liquid was cloudy throughout. A second test-tube containing the same 
* The process by which an atmospheric germ is wetted would be an interesting subject of investigation. A 
dry microscope covering-glass may be caused to float on water for a year. A sewing-needle may be similarly 
kept floating, though its specific gravity is nearly eight times that of water. Were it not for some specific 
relation between the matter of the germ and that of the liquid into which it falls, wetting would be simply 
impossible. Antecedent to all development there must be an interchange of matter between the germ and its 
environment ; and this interchange must obviously depend upon the character of the encompassing liquid. 
