APPENDIX E. cxxxi 



materials at the expense of which the ferment is enabled 

 to grow. Thus, then, just as the presence of a crystalline 

 fragment may determine the synthesis of its elements from 

 a solution in which they are contained 1 , and as the living 

 ferment may bring about that much more complex synthesis 

 which occurs during its growth, so may an organic poison 

 (having an intermediate molecular complexity) by its contact 

 with the fluids or mucous surfaces of the body, be enabled 

 to determine a series of changes leading to the synthesis of 

 a similar poison 2 . 



If we find that amongst this class of general or specific 

 diseases some are not at all, and others only slightly, con- 

 tagious, whilst the remainder present increasing degrees of 

 contagiousness; that diseases, which sometimes or under 

 some conditions are non-contagious, under others become 

 contagious; and lastly, if we find that even the virulently 

 contagious poisons of some diseases are undoubtedly 

 capable of arising de novo, then have we certain reasons 

 for the supposition, that the contagiousness or non-con- 

 tagiousness of particular general diseases is a quasi-acci- 



1 These elements, as Prof. Graham showed, really exist separately in 

 the solution, since they are separable by dialysis. 



2 Sir Thomas Watson says, in explanation of Liebig's doctrine, ' In 

 order, then, that a specific animal poison should effect its own repro- 

 duction in the blood, and excite that commotion in the system which 

 results from the formation and expulsion of the new virus, it is requisite 

 that a certain ingredient (analogous to the gluten in the brewer's wort) 

 should be present in the blood, and this ingredient must have a definite 

 relation to the given poison.' And he subsequently adds (' Principles 

 and Practice of Physic,' vol. ii. p. 790) : ' This theory of Liebig's offers, 

 then, an intelligible explanation of the curious facts that certain con- 

 tagious disorders furnish a protection, temporary or permanent, against 

 their own return ; that they have a tolerably definite period of incuba- 

 tion, and run, for the most part, a definite course ; that some persons 

 are less susceptible than others of the influence of these animal poisons, 

 or not susceptible at all ; and that the same individual may be capable 

 of taking a contagious disease at one time, and not at another.' The 

 same facts, it may be observed, are almost inexplicable in accordance 

 with any rational rendering of the ' germ-theory." 



i 2 



