88 FOURTH REPORT — 1834. 



is probably considerable. Its direct effects upon contagious ef- 

 fluvia are perhaps resolvable into temperature alone. 



XXXVIII. Such is a general outline of the facts that are 

 known respecting contagion, and of the conclusions to which 

 they lead. No one, however, who has inquired into this sub- 

 ject, can fail to be struck wiuh the imperfections of our know- 

 ledge respecting it, — with the paucity of observations sufficiently 

 correct to serve as the foundations of general laws, — and with 

 the number of questions which still remain to be solved*. A long 

 course of diligent attention to phaenomena, and a persevering 

 and rigid employment of the inductive logic, will doubtless sup- 

 ply many of these deficiencies. But there is another mode of in- 

 terrogating nature, hitherto little used in this department of phi- 

 losophical inquiry, that of experiment, which, in the investi- 

 gations of physiology, has supplied materials for the happiest 

 generalizations. In exploring the nature and laws of contagion, 

 experiment has hitherto done very little ; and extensive regions 

 of discovery remain to be entered upon, with the aid of that 

 powerful light. Difficulties and obstacles may be expected in 

 the research, but none that, either in number or amount, would 

 be insuperable by an ardent and inventive mind. Let it be re- 

 membered, as an incitement, that the inquiry has a higher ob- 

 ject than the gratification of speculative curiosity ; that its ten- 

 dency to the advantage of mankind is direct and unquestion- 

 able ; and that its success would add another triumph to those, 

 which philosophy has already achieved over physical evil, — evil, 

 no doubt, permitted to exist, among other reasons, that it may be 

 overcome by the vigorous use of those intellectual powers and 

 faculties, with which man is so preeminently endowed. 



XXXIX. This view of the subject of contagion would be in- 

 complete, without noticing a class of diseases, which have been 

 ascribed to causes of much more extensive operation, and are 

 generally contrasted with those of a contagious nature. They 

 are named endemic and epidemic DiSEASEsf. Both agree in 

 attacking a number of individuals ; but the former are more li- 

 mited than the latter as to the extent of their diflusion, and may 

 often be traced to causes of local operation. 



XL. 1 . Acute or febrile endemics prevail, either constantly or 

 periodically, over tracts of country of considerable area ; or they 

 may be confined to a province, a district, a city, or street, or a par- 



• As these questions arise obviously out of the statements of what is ah-eady 

 known, it appears unnecessary to collect them into a series of ' qitcBrenda.' 



f Endemic, from m in, and ovifio; the people ; Epidemic, from nrt tipon or 

 among, and the same subslantive. The terms, therefore, differ only in the 

 greater comprehensiveness given by the latter preposition. 



