But the cause of the disease is simply alluded to by this author, as depending upon "occult dispositions in the air," and as he speaks here of Measles with the Small Pox he goes on to say--"which necessarily cause these diseases and predispose bodies to them." This notion of Rhazes that there is some peculiar condition of the blood which favours a process resembling fermentation is not without interest. The circumstance that individuals are not {53} usually liable to a second attack of the disease, no doubt directed the attention of this physician to compare the process of fermentation with disease of such a nature, seeing that when the whole of the saccharine matter was converted into spirit, the hissing noise, as he calls it, or the disengagement of carbonic acid gas would cease, and the capacity for fermentation be entirely gone. So that the occult conditions of the air, their power of inducing a disease, and multiplying the matter capable of engendering a similar affection, stood in the mind of Rhazes as analogous if not identical phenomena. We pass now without further comment to the epidemics of the Middle Ages; and here the work of the philosophical Hecker leaves us little else to desire in the way of information, as far as it is obtainable from published records. From the manner in which he has grouped the facts which presented themselves to his mind in the course of a most laborious research, he has saved the student of this subject much toil in acquiring matter for reflection; he has here but to read and digest. I know not how to select from this invaluable work the most striking passages, to strengthen and support my hypothesis, for not a page is destitute of facts corroborative of the doctrine that vital germs are the material agents of pestilential disorders. The opening paragraph to the Black Death is a most cogent illustration of the assertion; it is, as it were, the theme of the work. "That {54} Omnipotence, which has called the world with all _its living creatures into one animated being_, especially reveals himself in the desolation of great pestilences. The powers of creation come into violent collision; the sultry dryness of the atmosphere; the subterranean thunders; the mist of overflowing waters are the harbingers of destruction. Nature is not satisfied with the ordinary alternations of life and death, and the destroying angel waves over man and beast his flaming sword." I must here apologise for large transcripts from Hecker's work, for neither could I command the amount of knowledge there displayed, nor use such appropriate language as the learned translator has employed.