REPORT ON THE LAWS OF CONTAGION. 93 



and supposed to form a distinct and separate class. But it must 

 not be forgotten that certain specific diseases, which by universal 

 consent are allowed to be contagious, at times prevail so gene- 

 rally as to be with propriety said to be epidemic. Such are the 

 smallpox, measles, scarlatina, and hooping-cough. But it is in- 

 conceivable that the specific poison, which in each of these in- 

 stances is the efficient cause of the disease, and which is the 

 undoubted product of vital operations, can be generated by any 

 ' corruption of air,' or by any spontaneous changes in inanimate 

 matter. The only way in which a general condition of the atmo- 

 sphere can be supposed to influence the spread of specific dis- 

 eases is, either by rendering it a better vehicle of their respective 

 poisons, or by influencing the predisposition of the body to re- 

 ceive them. But if the view which has been taken (§. XVII.) of 

 the state in which contagions exist in the atmosphere be correct, 

 temperature alone, by modifying the elasticity of those vapours, 

 can affiect their diffusion. It is well known, however, that ascer- 

 tainable conditions of the atmosphere, as to heat or cold, mois- 

 ture or dryness, and sudden transitions from the one state to its 

 opposite, produce in the animal body a predisposition to receive 

 contagion. The same atmospheric variations may act also as 

 exciting causes, calling into action contagious poisons already 

 admitted into the system, but not yet manifested by the usual 

 phsenomena ; and when they operate on numbers, may occasion 

 those sudden and violent outbursts of epidemic diseases, of which 

 several examples are on record. Other general influences, in- 

 deed, may prove exciting causes of such outbursts. They have 

 followed closely, for example, upon seasons of riot and intem- 

 perance, and have spread rapidly in situations where those dis- 

 eases were previously confined to few and scattered individuals. 

 It is equally unfavourable to the progress of knowledge to over- 

 estimate what we know, as to shrink from the just appreciation 

 of difficulties opposed to its further advancement. On the sub- 

 ject of epidemics, they who have inquired the most will be most 

 ready to admit, that our actual knowledge is bounded by very 

 narrow limits. But we are not on that account to despair. The 

 genius of philosophers of our own age has imfolded the most 

 astonishing truths with respect to the subtile agents — light, heat, 

 electricity, and magnetism. Every new conquest, that science 

 achieves, enlarges our powers over nature ; and we are fully en- 

 titled by the past to hope, that the physical condition of man will 

 in future be progressively improved by his acquiring a command 

 over external agents, which have never yet been subjected to his 

 knowledge and control. 



