152 LECTURES. 



by the action of nitric acid, mercury, and alcoliol, and used frequ^tly 

 in giving indications of aerial movements that would otherwise have 

 been invisible. Though the materials that became the principal object 

 of attention in ventilating operations were of great tenuity, it was 

 never to be forgotten that tliey might, in numerous respects, be treated 

 in the same way as water and other liquids. 



The quantity of air desirable for ventilation then came under con- 

 sideration. For each respiration the actual amount required was 

 small. From twenty to thirty cubic inches M-ere sufiicient for this 

 purpose ; but the expired air contaminates immediately a much larger 

 amount of the surrounding atmosphere. At the same time the sur- 

 face of the body is continually exhaling vitiated air in the same 

 manner as the lungs. Further, almost all kinds of clothing soon 

 become more or less charged with animal exhalations, and require 

 some addition to the ordinary supply, particularly if dyed with cer- 

 tain chemicals and exposed where they may have imbibed moisture. 

 It is also equally important to notice that every variety of tempera- 

 ture, electrical condition, and humidity in the atmosphere produces a 

 corresponding influence on the sensations as affected by the amount 

 of air brought in contact with the body in a given time. Further_, not 

 only are there great varieties of constitution in different individuals, 

 but even in the same person. Before and after dinner or any other 

 refreshment, before and after exercise, and under many other circum- 

 stances, very different quantities of air become agreeable or disagree- 

 able, and refreshing or oppressive. Lastly, minute and variable por- 

 tions of impurity from smoke and manufactories, or from terrestrial 

 exhalations, often modify the amount of supply that is desirable for 

 all constitutions. 



It will not be surprising, accordingly, that there is perhaps nothing 

 in respect to which there is a greater difference of practice than in the 

 amount of air given for ventilation, even where we assume that its 

 eifect is not still further modified by its mode of introduction and dis- 

 charge, and the efficiency with which it has the opportunity of acting 

 in passing through the apartment to be ventilated. 



It is surprising with how small a proportion of air existence can be 

 maintained for a long period when the system is comparatively inactive. 

 Dr. Reid then described an experiment, in which he had been hermeti- 

 cally inclosed in a case that was not broader than his shoulders, 

 deeper than his chest, or longer than himself; and stated that he had 

 continued there for upwards of an hour, the attendants being ordered 

 to take him out whenever he ceased to answer questions or to give 

 distinct replies. During the whole of that period he had not been 

 particularly incommoded, alter getting over a feeling of oppression 

 that attended his first respirations. Apprehensive, however, of 

 some subsequent injurious effects when the oppression he expected did 

 not increase so rapidly as he had anticipated, he directed the case to 

 be undone before any indications were given such as would have led 

 his assistants to have anticipated this order. Nor did he suffer so 

 much as he had expected from the effect subsequently, though head- 

 ache and restlessness continued for some days to a degree that {)revented 

 him from renewing his observations to the extent he had desired. 



