190 PROFESSOR LISTER. 



days ago I drew out milk from one of those that seemed to be 

 still pure, and I had the great satisfaction of finding it 

 not only perfectly fluid and tasting perfectly sweet, with a per- 

 fectly normal reaction, purpling both blue litmus paper and 

 red litmus paper — the norm alreaction of perfectly fresh milk 

 — but under the microscope I could not discover any organism 

 of any kind whatsoever. Therefore, I think we are justified in 

 saying that in unboiled milk as in boiled milk, provided, of 

 course, the cow be healthy, there does not exist any constituent 

 having the power of giving rise to organisms or producing the 

 lactic or any other fermentative change. 



This, gentlemen, was the first step of the investigation : to the 

 second I must beg your special attention, because I bel eve yua 

 will agree with me that it is the far more important step of the two. 



The object of the second part of the investigation was to find 

 absolute evidence, if possible, whether the Bacterium lactis was 

 or was not the cause of the lactic fermentation. It occurred to 

 me that, if we could estimate with some degree of accuracy the 

 number of bacteria present in a given quantity of souring milk, 

 and then if we were to dilute the milk with a proportionate 

 quantity of boiled water, we might have the diluted milk so 

 arranged that every drop with which we should inoculate boiled 

 milk might contain, on the average, one bacterium ; and if we 

 should do so, as it would be practically certain that the bacteria 

 would not be distributed with absolute uniformity, we should 

 expect that we might have, as the result of these various inocula- 

 tions, some glasses with the lactic fermentation, some glasses 

 without it, some with the Bacterium lactis, and some without it ; 

 and, if it should turn out that those glasses which underwent 

 lactic fermentation should all contain the Bacterium lactis, and, 

 on the other hand, those glasses which had no fermentation 

 should be free from bacteria, that would prove the point ; as, I 

 think, you will agree with me, when we come to discuss the matter 

 at a little more length after we have all our facts before us. Well, 

 how were we to determine the number of bacteria existing in the 

 liquid ? This was done in a simple manner. A circular covering 

 glass, just half an inch in diameter, was used. Of course, we 

 know how many square thousandths of an inch there are in the 

 area of this little glass. We also know by the micrometer how 

 many thousandths we have across the field of our micro- 

 scope, and, therefore, by calculation we know how many square 

 thousandths there are in our field, and thus we can tell how many 

 fields there are in the covering glass. To measure the liquid, I 

 used this little syringe, with the piston rod in the form of a screw, 

 on which resolves a disc, graduated for lOOths of a minim, by 

 which means you can, with perfect precision, emit 1-lOOth of a 

 minim, or 2-lOOths, or any number you choose. I found that 



