PROGRESS OF MICROSCOPICAL SCIENCE. 229 



minutes, in flasks previously plugged with cotton wool, remained per- 

 manently barren. This result was absolutely invariable. 



II. More complex mixtures — milk, neutralized or alkalized infusions 

 of vegetable and animal tissues, similar albuminous and gelatinous 

 solutions, mixtures containing fragments of animal or vegetable sub- 

 stances or cheese — yielded variable results. In none of them did 

 fungoid growths make their appearance — but monads and bacteria 

 frequently appeared in abundance. 



This seemingly contradictory result was inferred to be due to the 

 ineffective application of the heat in the process of direct boiling over 

 a flame. It was found that many of these more complex mixtures 

 frothed excessively when boiled — brisk ebidlition could not therefore be 

 maintained — particles were spurted about on the sides of the flask, and, 

 in this way, apparently escaped effective exposure to the heat. Even 

 when the boiling was prolonged for 20 or 30 minutes the results were 

 still uncertain — sometimes the flasks remained barren — sometimes 

 they became turbid and swarmed with bacteria. 



III. By the second modification of the experiment much more 

 constant results were obtained — the flasks remained almost always per- 

 manently barren — and the few exceptions were found to be due to some 

 imperfection in the conduct of 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 sub- 

 stances 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 comminuted, 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 experi- 

 ments 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 concen- 

 tration, 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 withdrawn 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 mil- 

 dew 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 its dust, and not in its 

 gaseous ingredients ; but as to the exact nature of that biogenic power 

 — whether it be a specific germ or a ferment — no sufficient evidence 

 has yet been adduced. Dr. Roberts did not find that diminished 



