448 



believe that aqueous vapour is its ordinary vehicle of commu- 

 nication. Of its occasional sources, the temperatures most 

 favourable to its evolution, the laws which seem to govern 

 its propagation, the extrinsic agencies which modify its 

 action on the animal economy, and of the measures to be 

 resorted to to guard against its absorption, or to render it 

 inert, we are not without light to guide us. But whether it 

 is always one and the same poison, modified only in its 

 operation by its degree of concentration, by caloric, 

 electricity, and the peculiar constitution of the infected, or 

 that there are diversities of malarious virus, is not positively 

 ascertained ; though I myself decidedly incline to the latter 

 opinion as best supported by corroborative evidence. Like 

 will produce like, as the seeds of plants reproduce seeds, 

 and the poisonous vesicle or pustule generated in small pox, 

 cow pox, plague, and syphilis, develop these diseases, and no 

 other. 



Malaria, in its medical sense, though often associated with 

 stench, is by no means necessarily so ; neither is the presence 

 of animal and vegetable putrefaction essential to its genera- 

 tion ; though in towns we find abundance of it in the vicinity 

 of obstructed and ill-conditioned sewers. It is more than 

 probable that offensive odors promote its absorption into the 

 human system by their known sedative action ; but in the 

 original specific poisons of marsh miasmata, of the Asiatic 

 cholera, influenza, and epidemics generally, we are not 

 conscious of any smell except that which is common to the 

 districts in which they are chiefly found to prevail. Atmos- 

 pheric dilution commonly diminishes the virulence of aerial 

 poison, and the latest seizures from its efi'ects usually assume 

 a milder form of attack, as though it had spent its powers, or 

 a change had been eff'ected in the electric and atmospheric 

 currents. Actual contact with the infected, in the most 

 avowedly contagious diseases, does not necessarily induce 



