171 



the measurement of the purity of the air, is far from accomplishing what 

 its name indicates, and has disappointed the hopes which had been en- 

 tertained on the subject. Eudiometrical instruments can inform us, on- 

 ly of the proportion of oxygen contained in the atmosphere; now, its sa- 

 lubrity, its fitness for respiration, is not in proportion to the quantity of 

 oxygen. The volatilized remains of putrid animal or vegetable substan- 

 ces, various mephitic gases, combine with it, and affect its purity. In 

 the comparative analysis of air procured on the Alps and in the mar- 

 shes of Lombardy, there was found in each the same quantity of oxygen; 

 and yet, those who breathe the former, enjoy robust health, while the 

 inhabitants of the marshy plains of Lombardy are carried off by epidemic 

 diseases, are pale, emaciated, and habitually lead a languid existence. 



Though, at least, 0,20 of oxygen are necessary to render the air fit for 

 respiration, the proportion may be diminished to seven or eight parts in 

 the hundred ; but in such cases, the breathing is laborious, panting, and 

 attended with a sense of suffocation, in short, asphyxia comes on, even 

 while the air stil! contains a certain quantity of oxygen, of which the lungs 

 cannot entirely deprive it. Whenever a number of persons are collected 

 in a confined place, in which the air cannot be easily renewed, the quan- 

 tity of oxygen diminishes rapidly, that of carbonic acid increases. The 

 latter, in consequence of hs specific gravity, sinks to the lowest part, and 

 strikes with death every living being which it envelopes. When two 

 lighted candles, of different lengths, are placed under the same bell, the 

 shorter candle goes out first, because the carbonic acid formed during 

 combustion, sinks to the most depending part. For the same reason, the 

 pit is the most unhealthy part of a play-house, when a great number of 

 people after remaining in it for several hours, have deprived the air of a 

 cousiderable portion of its oxygen. 



Persons collected together, and enclosed in a small space, injure each 

 other, not only by depriving the atmosphere of its respirable element, but 

 particularly by altering its composition, by the combination of all the sub- 

 stances exhaled from their bodies. These volatilized animal emanations, 

 become putrid, while in the atmosphere, and conveyed to the lungs dur- 

 ring respiration, becomes the germ of the most fatal diseases. It is in this 

 manner, that the jail and hospital fevers so fatal to almost all whom it at- 

 tacks, arises and spreads. A dry and temperate air containing 0.27 of 

 oxygen and 0.73 of azote, and free of other gases, or other volatilized 

 substances, is the fittest for respiration. In certain cases of disease, how- 

 ever, this function is most freely performed in a less pure air. Thus, 

 patients labouring under pulmonary consumption, prefer the thick and 

 damp air of low situations, to the sharp and dry air of mountains; nerv- 

 ous women prefer that in which horn, feathers, or other animal substances 

 are burning. An atmosphere highly electrical, at the approach of a storm, 

 renders respiration very laborious, in some cases of asthma. In short, 



thollet, that the atmosphere contains only twenty-two parts of oxygen, in the hundred, 

 by measure. 



By some chemists it has been supposed, from the circumstance of the carbonic acid 

 being generally found in a larger quantity near the earth, that it is an accidental, and 

 not an essential constituent of the atmosphere. But by De Saussure it was detected in 

 the air, on the summit of Mont Blanc ; and from this, and a variety of other considera- 

 tions, it would appear to be a uniform part of atmospheric air, existing most probably 

 in a state of chemical combination. Chapman. 



