54 
the experiment. No exceptions occurred with milk, nor 
with substances, however complex, which were in actual 
solution, but when considerable pieces of vegetable or animal 
substances were introduced into the flasks, bacteria and 
monads with putrefactive changes occasionally made their 
appearance in abundance. In these exceptional cases, when 
the experiments were repeated with the pieces finely com- 
• 
minuted, or introduced in some other way more favourable 
to the diffusion of the heat, the flasks remained permanently 
barren. 
Dr. Roberts called attention to the crucial significance of 
experiments on this subject made in flasks whose necks are 
plugged with cotton wool. A plug of cotton wool acts as 
an absolutely impervious filter to the solid particles of the 
atmosphere, while it permits a free passage to the gaseous 
constituents. 
When one of these experiments is effectively performed, 
the fluid or mixture in the flask may be exposed to the full 
influence of light, of warmth, and of air, and yet it remains 
permanently barren. As slow evaporation takes place the 
liquid passes through all grades of concentration, possibly 
chemical changes of various kinds take place within it, and 
still no organic growth makes its appearance for months 
and even years ; but if the plug of cotton wool be with- 
drawn for a few minutes, or a single drop of any natural 
water, however pure and well filtered, be introduced, then 
all is changed— in a few days the clear solution becomes 
turbid from bacteria and monads, or a mass of mildew covers 
its surface and soon half fills the flask. 
In the face of these experiments it was impossible to 
doubt that the biogenic power of the atmosphere resides in 
