184 Dr. G. Child on the Production of [Apr. 27, 



Now, it may be asked, why the same or similar organisms were not 

 found in the other cases, if the experiments were fairly tried ? The answer 

 is this, viz. that we do not know all the conditions under which they exist. 

 It is pretty clear that they appear more easily in some substances than in 

 others. Thus, in the first series above described, it will be noticed that the 

 four instances in which none were found were all those in which coarse 

 flour was the substance used. In the remaining three, where pea-meal or 

 hay were employed, there the bacteriums were seen. So also in the other 

 series, the one case in which nothing was found was a case in which flour 

 was used, and in the remaining five the most numerous and distinct 

 bacteriums were seen in the hay infusion. This may arise possibly from the 

 fact that the infusion of flour is not so clear as the others, and always con- 

 tains more granular matter ; thus bacteriums are less easily distinguished 

 in it : and, where doubtful, it is my practice to decide in the negative ; that 

 is to say, unless the bacteriums are clearly seen, I enumerate the experi- 

 ment amongst those in which they are not found. Further, it is possible 

 that in some infusions they may live and die sooner than in others, and in 

 most of these experiments with flour there was a mass of indeterminate 

 granular matter which might have contained the bodies of whole popula- 

 tions of bacteriums. Finally, it is quite possible that they might, if exist- 

 ing in small numbers, escape observation. Their minuteness is extreme, 

 and observation of them far from easy. At any rate, positive evidence in 

 a matter of this kind is of more value than negative ; and the fact that in 

 eight cases out of thirteen they have been seen, not by myself only, but also 

 by so accurate and practised a microscopist as Dr. Beale, is of more weight 

 than our having been unable to discover them in the remaining five cases. 



The question which now remains to be discussed is, how it is that the 

 results above given so entirely disagree with those arrived at by M. Pasteur, 

 and now, to a certain extent, vouched for by the Commission of the Academy 

 of Sciences. I have observed all the precautions which M. Pasteur him- 

 self speaks of as " exaggerated," yet I have shown bacteriums to be pro- 

 duced exactly under the circumstances in which he asserts that they do not 

 exist. I believe this discrepancy is very easily accounted for. M. Pasteur, 

 in his memoir, speaks of examining his substances with a power of 350 

 diameters. Now my experience throughout has been that it is impossible 

 to recognize these minute objects, with any degree of certainty, even with 

 double that magnifying power. When once their existence on a slide is 

 shown with a power of 1500 to 1700 diameters, it is quite possible after- 

 wards to recognize the same object with a power of 750, but I have re- 

 peatedly failed to satisfy myself in the first instance with the latter power ; 

 and on the one occasion on which I enjoyed the use of an object-glass giving 

 a power of 3000 diameters, I found the recognition of these very minute 

 objects rendered very much more easy. On one occasion 1 tried the effect 

 of a power of 450 (not possessing one of 350), and found that all satisfac- 

 tory investigation of such objects with such a power was impossible. Any 



