in relation (o Disease. 30 



these miasmata as the cause of disease. It is idle to speculate 

 upon physical relation here (whether tliat of solution, or of 

 independent elasticity, according to Dalton^s theory of va- 

 pours), while so entirely ignorant of the chemical constitution 

 of these agents. We can only affirm that the conditions which 

 concur to their production, are likely to aid their diffiision and 

 action on other bodies : and though the proofs are by no means 

 assured, yet there is evidence that a foggy and humid state of 

 atmosphere is that in which contagious or epidemic diseases 

 are most readily and extensively spread. Other causes, hpw- 

 ever, doubtless operate, and produce many apparent or ireal 

 exceptions to the fact. 



Recurring to the more direct influence of air, loaded with 

 moisture, on the body, we have reason to expect it to be great- 

 est on the functions of respiration and of the skin ; and obser- 

 vation, as far as it goes, confirms this view. The effects in 

 each case are probably owing chiefly to the altered amount of 

 discharge from the exhalant vessels of the organs concerned ; 

 in part also, especially when the external temperature is low, 

 to the greater effect of cold, conjoined with moisture, on the 

 capillary vessejs and sentient ei^tremities of nerves of the 

 surface exposed. The difference to the feelings between a 

 temperature of 45° Fahr., in dry or in damp air, is one which 

 cannot escape the most ordinary attention. It is a difference 

 equally marked as that between steam, and air heated to 212°, 

 in their respective application to the body. The membrane 

 lining the air passages is obviously most liable to these effects 

 and to disorders depending upon them ; as the experience pf 

 patients suff'ering under asthma and bronchitis, however vary- 

 ing in details, painfully testifies in its general results. 



For the reasons already given, we are rarely indeed entitled 

 to speak of humidity alone as a morbid cause ; but it undoubt- 

 edly concurs with and renders others more effectual. And in 

 the case of very damp air received into the lungs, it is proba- 

 ble that it may act expressly by retarding or impairing the 

 changes made in respiration ; and especially those depending 

 on exhalation, which form so important a part of this process. 

 Modern research, in shewing the facility with whicji tli^so 

 changes take plaoe (not merely by vascular structure, bui 



