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 difiusion 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 flltered, 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 



