184 PROFESSOR LISTER. 



portions of the three elements, carbon, hydrogen, and oxygen, 

 remains identically the same ; but those of you who are chemists 

 understand what I mean when I say the atomic weight of the 

 lactic acid is one fourth of the atomic weight of the sugar of 

 milk. Each atom of milk sugar is resolved into four simpler 

 atoms of lactic acid. Now, it may be naturally supposed, if you 

 observe what happens in a portion of milk obtained from a dairy, 

 that there is an inherent tendency in the milk to this souring and 

 curdling. If you get milk from a dairy and keep it long enough, 

 it is certain to turn sour and curdle, then, after a while, there 

 comes a certain mould upon the surface, the O'kVmm lactis, which 

 constitutes the sort of bloom there is upon a cream cheese ; then 

 comes on, often simultaneously with the growth of this mould, 

 the butyric fermentation, in which butyric acid is produced ; and 

 afterwards, if you keep the milk long enough, it will probably 

 putrefy. When you see, time after time, specimens of milk, 

 taken from various dairies, undergo this succession of alterations, 

 you may be tempted to suppose that these were clianges to 

 which the milk was disposed from its own inherent properties as 

 it comes from the cow^s udder. The late eminent Professor of 

 Chemistry in this College, Professor Miller, in his excellent work 

 on Chemistry, states that the ferment of the lactic acid fermen- 

 tation is the caseine of the milk. I am bound to say, however, 

 injustice to Professor Miller, that he also adds that M. Pasteur 

 has expressed his belief that there exists an organic living fer- 

 ment which produces this fermentation; but Professor Miller 

 does not profess to decide between these two opinions. On the 

 contrary, his first statement, that the caseine is the ferment, 

 might lead you to sup])ose that he is inclined to the former 

 view.^ If this were the case, as there is caseine always in the 

 milk, there should always be the lactic acid fermentation. But 

 it was pointed out long ago by M. Pasteur that, if you examine 

 any specimens of souring milk with the microscope, you find 

 little organisms.2 These, when you come to look at them care- 

 fully, you see to be obviously of the nature of bacteria. Bac- 

 teria may either have the faculty of motion or they may not. 

 This particular bacterium is a motionless bacterium, so far as I 

 know; still it has the essential nature of a bacterium : a micro- 

 scopic fungus, multijjliod by lissiparous generation, the lines of seg- 

 mentation being transverse to the longitudinal axis of the organism. 

 I have ventured to give to this little organism the name Bacterium 

 lactis; for, gentlemen, no doubt there are difl'erent kinds of 



' Vide Miller's' 'Elements of Cliemistry,' third edition, vol. iii. 

 ^ Vide " Memoire sur la Fermentation appolee Lactique," ' Annates de 

 Cluinie ct de Physique,' 3me s6rie, tome Iii, 1858. 



