IN MEDICAL SCIENCE. 11 



been just the other way, salt in prisms and saltpetre 

 in cubes, or why either should take an exact geometri- 

 cal outhne, any more than coagulating albumen. 



But although we had given up attempting to explain 

 the essential nature of affinities and of crystalline types, 

 we might have supposed that we had at least fixed the 

 identity of the substances with which we deal, and de- 

 termined the laws of their combination. All at once 

 we find that a simple substance changes face, puts off its 

 characteristic qualities and resumes them at will; — not 

 merely when we liquefy or vaporize a solid, or reverse 

 the process ; but that a solid is literally transformed 

 into another solid under our own eyes. We thought 

 we knew phosphorus. We warm a portion of it sealed 

 in an empty tube, for about a week. It has become a 

 brown infusible substance, which does not shine in the 

 dark nor oxidate in the air. We heat it to 500° F., 

 and it becomes common phosphorus again. We trans- 

 mute sulphur in the same singular way. Nature, you 

 know, gives us carbon in the shape of coal and in that 

 of the diamond. It is easy to call these changes by the 

 name allotropism, but not the less do they confound our 

 hasty generalizations. 



These facts of allotropism have some corollaries con- 

 nected with them rather startling to us of the nine- 

 teenth century. There may be other transmutations 

 possible besides those of phosphorus and sulphur. 

 When Dr. Prout, in 1840, talked about azote and car- 

 bon being "formed" in the hving system, it was 



