A third treatise of considerable importance is that which is devoted by Rhazes to the description and treatment of small-pox and measles. So far as is known at the present time this is the first treatise that has been written on these diseases, and its celebrity rests, not only upon this circumstance, but also upon the facts that its author is evidently familiar with the different types of small-pox and with the characteristic features which distinguish this disease from measles. Freind, in commenting upon this treatise, says that Rhazes assigned for small-pox a cause “entirely new in physick, a sort of an _innate contagion_. This is a _ferment_ in the blood, like that in must, which purifies itself sooner or later by throwing off the peccant matter at the glands of the skin; an hypothesis since applied, though upon very slight grounds, to feavers in general by many moderns.” From this account it is fair to conclude that Rhazes, in the tenth century of the Christian era, as clearly suspected the germ origin of certain febrile diseases as Liebermeister did toward the end of the nineteenth, or as Fracastoro did in the sixteenth. And one cannot help exclaiming: How many centuries had to elapse, and what an immense amount of other facts had still to be discovered--facts in anatomy, in physiology, in chemistry, in optics, etc.--before it became possible to convert this suspicion, this simple product of the reasoning faculty, into an actual demonstration of the truth in pathology!