528 Professor Percy F. Franhland [Feb. 19, 



To micro-organisms again then we must ascribe the accomplish- 

 ment of this highly important chemical change going on in the soil, 

 although it has not hitherto been so fully illuminated as the process 

 of nitrification. 



Selective Action of Micro- organisms. 



Any of the ordinary plants and animals with which we are 

 familiar may be regarded as analytical machines^ and we ourselves, 

 without any knowledge of chemistry, are constantly performing 

 analytical tests ; thus we can all distinguish between sugar and salt 

 by the taste, between ammonia and vinegar by the smell, whilst by a 

 more elaborate investigation we distinguish, for instance, between the 

 milk supplied from two different dairies by ascertaining on w^hich we 

 or our children thrive best. In fact, such analytical or selective 

 operations are amongst the first vital phenomena exhibited by an 

 organism on coming into this world. It is, however, particularly 

 surprising to find this analytic or distinguishing capacity developed 

 in an extraordinarily high degree amongst micro-organisms. From 

 the power which we have seen that some possess of flourishing on the 

 extremely thin diet to be found in distilled water, we should be 

 rather disposed to think that caprice would be the very last failing 

 with which they would be chargeable. As a matter of fact, however, 

 the perfectly unfathomable and inscrutable caprice of these minute 

 creatures is amongst the first things with which the student of 

 bacteriological phenomena becomes impressed. Let me call your 

 attention to a striking example of this which I have recently investi- 

 gated. 



I have here two substances, which have the greatest similarity : — 



Mannite. Dtjlcite. 



Occurrence. Numerous plant-juices .. .. Ditto, but less frequently. 



Taste. Sweet Ditto, but less so. 



Melts. 166° C 188° C. 



Crystalline form. Large rhombic prisms . . Large monoclinic prisms. 



Not only, however, do these two substances possess such a strong 

 external resemblance to each other, but in their chemical behaviour 

 also they are so closely allied that one formula has to do duty for 

 both of them, for so slight is the difference in the manner in 

 which their component atoms are arranged that chemists have not 

 yet been able with certainty to ascertain in what that difference con- 

 sists. Under these circumstances it would have been anticipated that 

 bacteria would be quite indifferent as to which of these two sub- 

 stances was presented to them, and that they would regard either 

 both or neither as acceptable. But such is by no means the case); 

 some micro-organisms, like ordinary yeast, have no action upon either, 

 whilst others will attack mannite, leaving didcite untouched, others again 



