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them, nor does the absence of this condition confer an 

 exemption from attack ; and hence it is often wise in those 

 who have no family ties or social obligations on the spot, 

 to fly from the scene of any spreading pestilence, whilst for 

 those who remain the best protection will be found in the 

 early adoption of judicious sanitary precautions. 



Nature's own atmosphere is, I apprehend, the only 

 gaseous medium consistent with the maintenance of health. 

 All deviations, therefore, in its chemical composition and the 

 relative proportions of its own constituent principles, as well 

 as its admixture with foreign solid and aeriform fluids, must 

 be regarded as more or less injurious, according to the degree 

 of variation and their peculiar mode of action. Hence every 

 gas, other than those contained in the atmosphere, may be 

 viewed as poisonous, and unfitted for the due aeration of the 

 blood in its circulation through the lungs. This explanation, 

 if admitted, will enable me to simplify the subject, and dis- 

 pense with any detailed classification of the deleterious gases 

 founded on their diflPerent properties and manner of operation. 

 Some of them, when concentrated or pure, are for a time 

 respirable; others are irrespirable in such a state; some, even 

 when largely diluted with common air, are narcotic in their 

 action ; and others of them thus circumstanced are irritants. 

 I do not think there are any possessing negative qualities, 

 although usually so divided. Nitrogen itself, when pure, 

 will soon induce suff'ocation ; and its greater affinity for 

 hydrogen makes it readily quit the atmosphere to form 

 ammonia and cyanogen. If superadded to the air beyond 

 nature's proportions, the effect will be to exclude one-fourth 

 of its own volume of oxygen in any present given quantity, 

 which could not fail to be pernicious if inhalation of it were 

 long continued, for oxygen undoubtedly performs the prin- 

 cipal functions in the process of respiration. Hydrogen, 

 like nitrogen, may be inhaled for a very short time, but in 



