%&& Progress of Rational Pathology. 



of the skin. — The physico-chemical properties of conta- 

 gions shew, that their matter must be organic. From the 

 way in which they act, it follows that the matter of contagion 

 is alive, and indeed endowed with individual life, and that it 

 bears to the diseased body the relation of a parasitic orga- 

 nisation. Contagions, as is proved by the course they run, 

 and by the phenomena of the illness produced by them, 

 possess two properties which do not belong to any form- 

 less or dead matter : the power of multiplying themselves 

 on food, and by the assimilation of foreign organised bodies, 

 and a certain power of periodical development limited by cer- 

 tain conditions, (they either run a chronic course, or they 

 have a fixed succession of stages.) The spreading of con- 

 tagions on the diseased body is thus either limited or unlimited : 

 all contagions have a mode of propagation, which corresponds 

 with that of higher organisations. We do not need to assume 

 equivocal generation, if we admit that the germs of infectious 

 matter may lie, out of the diseased body, as the germs of the 

 lower organisations do, out of fermenting and putrefying sub- 

 stances, in a state capable of development, and only wanting 

 favourable conditions for their propagation. These conditions 

 appear to be : external ones, such as certain atmospheric re- 

 lations : and internal ones, such as changes in the body, which 

 render it fit for the support of parasites. In the same 

 way, the conversion of merely miasmatic into contagious 

 diseases, if the fact were once indubitably proved, could 

 be explained without the aid of equivocal generation. Purely 

 miasmatic disease, such as ague, catarrh, diarrhoea, &c. 

 would be the means by which the human frame is rendered 

 capable of receiving the infectious matter, which when once 

 taken in, is propagated in the form of contagion. Thus it is only 

 the origin of infectious matter and of contagion that remains 

 hidden ; but this is the case with all organisations, nay with all 

 matter. — It remains now to examine, whether contagion be 

 conditionally or absolutely independent, (of vegetable or of 



