150 * LECTURES. 



tions in cities, villages and populous districts, as well as the means 

 of correcting them. It was a self-evident proposition that the first 

 step in all effective ventihition is to start with a good atmosphere; 

 but such was the apathy, indifference, and sometimes the ignorance, 

 on this point that it often became a most troublesome question to deal 

 witli in a satisfactory manner, particularly where tracts of ground 

 had become saturated with debris in a perpetual state of putrefactive 

 fermentation, or where streams or stagnant water were loaded with 

 similar materials. In the great theatre of the globe itself, the gen- 

 eral purity of the atmosphere was sustained by the mutual relations 

 of the animal, the vegetable, and the mineral kingdom ; by the per- 

 petual rotatory currents flowing from the equator toward the poles 

 and from the poles towards the equator ; by that great peculiarity in 

 all gases and vapors which constantly led to their diffusion through 

 each other, however different in specific gravity, so that nowhere on 

 the surface of the earth where there was free access to the external 

 atmosphere could any accumulation of any noxious product take place 

 without a process of dissipation and dilution being immediately com- 

 menced ; and by the chemical action of tlie air, which was perpetually 

 tending to oxidate or burn all malarious products. But how largely 

 were these natural agencies counteracted, within as well as without 

 doors, when there was a deficiency in the supply of air, or an excess 

 in the material of decomposition. Many were the districts in which 

 a rich and luxuriant vegetation consumed the products that gave rise 

 previously to fever and ague. Travellers have expressed their great 

 surprise at the total absence of these diseases under circumstances 

 where they had anticipated their severe operation, and traced, subse- 

 quently, to the action of special plants the conservative influence that 

 guarded them from danger. Let this lesson, said Dr. Keid, not be 

 neglected ; let it be applied in full force, and the facts be studied 

 and developed with an untiring assiduity, till miasma shall be largely 

 overcome in all cities subject to its influence, and the water-lilly and 

 other aquatic plants shall have improved the condition of all accu- 

 mulations of water in their vicinity, as much as an active and vig- 

 orous vegetation purifies the air that moves upon the land. If he 

 dwelt more upon this point than might at first appear requisite, it 

 was because its importance, though admitted, was by no means ade- 

 quately estimated. He did not consider that there was any question 

 connected with the material world that promised greater blessings to 

 large cities and populous districts tlian those that would flow from 

 professional investigation and practical experience in this department, 

 combined with the information available from former ages, and the 

 practice of different nations. It had been demonstrated that a large 

 proportion of the deaths that filled the annual bills of mortality arose 

 from ])reventible causes ; and in making any estimate on this subject, 

 it ought never to be forgotten that every death indicated many cases 

 of disease and suffering that were never registered in the ordinary 

 tables. How great, then, is the question at issue, and how man}^ and 

 how varied would the channels be through which its right solution 

 would affect society ? 



Dr. Reid then showed by experiments the fundamental principles 



