72 FOURTH REPORT 1834. 



VIII. It may be held as a general principle, that no specific 

 poison ever gives rise to any other contagious malady, than that 

 of which it is itself the product. The poison of smallpox never 

 occasions measles ; nor that of measles smallpox. It must 

 however be acknowledged, that the sequent disease is seldom an 

 exact /ac simile of the antecedent, but often differs from it, not 

 only in degree, but in the absence of one or more of the usual 

 pheenomena, or in the addition of others not commonly ob- 

 served. Scarlatina, it is well known, when connnunicated to 

 numbers from a common source, may affect some severely and 

 others slightly ; and the general fever, the eruption, and the af- 

 rection of the fauces and throat, may exhibit almost infinite va- 

 rieties. In like manner, the mild and distinct smallpox has 

 often imparted a confluent and dangerous sorf ; and the reverse. 

 It is needless to mvdtiply examples, because inconstancy of 

 symptoms is observable, not of contagious disorders only, but 

 of all others, whether acute or chronic. Our classifications and 

 nomenclatures of diseases are in fact founded, not on constant 

 and uniform characters, like those establishing the distinctions 

 of natural history, but on general features, which are liable to 

 be qualified by many exceptions, and which present almost in- 

 finite varieties of aspect. 



IX. Of the nature of those processes, by M'hich a simple fever 

 becomes contagious in its progress, we are totally ignorant. The 

 opinion that. a contagious poison is, in any case, generated by a 

 change in the animal fluids analogous to fermentation or to putre- 

 faction, (a change veiled by Sydenham under the phrase commo- 

 tio sangtmiis), is inconsistent with general reasoning as well as 

 with observation. The tendency to putrescence in the solids or 

 fluids of the animal body, at temperatures favourable to that pro- 

 cess in dead matter, is counteracted by the undefinable principle 

 of LIFE, so long as that principle retains sufficient energy. During 

 a contagious fever, none of those gases are iiecessarily evolved, 

 which are the constant products of animal putrefaction. A person 

 sick of typhus fever, enjoying all the advantages of cleanliness and 

 fresh air, and emitting no sensible odour, may yet impart a fatal 

 infection. An instance is on record, in which a person under such 

 circumstances was accompanied, for about half a mile, in a 

 coach, by four individuals, none of whom perceived the slightest 

 odour, but all caught the infection, and died in consequence*. 

 It may be remarked also, that the odours, which arise from per- 

 sons labouring under acute specific diseases, are not similar to 

 those of common putrefying matter, but are distinct and pe- 



• Fordyce, Dissertation, p. 115. 



