CONDITIONS FAVOURING FEEMENTATION. 33 



batcli of 6-10 retorts or flasks whose contents had been boiled 

 over the flame for 2" and which were sealed during ebullition, I 

 have correctly predetermined that this particular fluid should re- 

 main sterile by simply omitting to break its associated liquor- 

 potassse tube, and that the others should ferment even after boiling 

 them for an additional period four times as long — this result 

 being induced simply by breaking their properly charged and 

 superheated liquor-potassse tubes. It seems, therefore, super- 

 fluous to urge that when such experiments are multiplied with 

 essentially similar results, as they have been, they afl'ord evidence 

 of the most crucial and unmistakable character. 



Dr. Eoberts, however, impeaches this well-tried experimental 

 " method of difierence ; " and Professor Tyndall countenances the 

 impeachment. Let us see what kind of method they vrould put 

 in its place. 



Dr. Eoberts thinks it absolutely necessary to wait several days, 

 or even a fortnight, in order to make perfectly sure that each in- 

 dividual fluid is sterilized before adding the superheated potash 

 thereto. Having shown that the grounds on which he rejects 

 the evidence of the control experiment are peculiarly weak in 

 regard to a fluid so easily sterilized as urine, I wdll now endeavour 

 to show that the alternative procedure which he recommends to 

 replace this legitimate method is, on the contrary, open to the 

 most serious objections. Dr. Eoberts, like Professor Tyndall, is 

 never unmindful of " the plain indications of the germ theory ; " 

 and though he does not persistently ignore, still he is not always 

 equally mindful of the plain indications of the opposite theory. 

 From the point of view of the germ theory, it is true, there is not 

 so much to be said against this delay in the liberation of the 

 liquor potassse ; but from the point of view of the opposite theory, 

 the serious question arises as to whether any or what chemical 

 changes would take place in the experimental fluids during this 

 period of probation. Other questions, as will be seen, have also 

 to be considered in regard to their method. 



It is almost difiicult to believe that Dr. Eoberts could have 

 been speaking seriously when he said* that from the point of view 

 of the spontaneous-generation theory " there was no reason why 

 the alkali should not have been equally effective in promoting 

 germination, whether added before or after the short preliminary 



* Phil. Trans, vol. clxiv. (1874) pt. 2, p. 474 

 LINN. JOUHN. — ZOOLO&Y, VOL. XIY. 3 



