236 MEDICINE 



The microbic origin of disease is a fact of 

 nature which might well have for ever eluded 

 the methods of clinical observation and morbid 

 anatomy. It could not possibly elude the methods 

 of Pasteur. 



Pasteur's first researches dealt with a region 

 of fact which seemed remote enough from the 

 interests of practical medicine, though a region 

 of great importance to science. It was already 

 known to chemists that certain substances exist 

 in two distinct, though closely related, forms, 

 identical in all their properties save that the one 

 form in transmitting polarised light twists the ray 

 to the right, while the other form twists it to the 

 left. As a crystallographer Pasteur discovered that 

 in such cases the crystals of the one form differ 

 from those of the other with that kind of difference 

 which is illustrated by the two hands of a man 

 essentially identical but not superimposable one 

 of them corresponding to the mirror image of the 

 other. Sometimes a substance capable of existing 

 in two such forms may, when in solution, show 

 no action upon a ray of light. In such a case we 

 now speak of the substance as being in the racemic 

 form. In reality, as Pasteur showed, both active 

 forms are then present, and these neutralise each 

 other. We are now to see how a crystallographer, 

 knowing little or nothing of medicine, came to 

 inaugurate one of the greatest advances in our 

 knowledge of disease. 



