EXHALATION OF WATEKY VAPOR. 153 



in respiration are not favorable to the hypothesis that the carbonic acid is the result of a 

 direct action of oxygen upon carbonaceous matters. We should hardly expect a definite 

 relation to exist between these two gases in respiration, when we find carbonic acid ex- 

 haled in the absence of oxygen. 



Many of the points which we have considered with relation to the variations in the 

 exhalation of carbonic acid have been investigated by experiments in Pettenkofer's cham- 

 ber, and the results very nearly correspond with the observations of Scharling, Smith, and 

 others which we have quoted. 



Sources of Carbonic Acid in the Expired Air. All the carbonic acid in the expired 

 air comes from the venous blood, where it exists in two forms; in a free state in simple 

 solution, or at least in a state of very feeble combination, and in union with bases, form- 

 ing the carbonates and bicarbonates. That which exists in solution in the blood is simply 

 exhaled. The alkaline carbonates and bicarbonates of the blood, coming to the lungs, 

 meet with pneumic acid (discovered by Verdeil in 1851), and are decomposed, giving rise 

 to a farther evolution of gas. It is pneumic acid which gives the constant acid reaction 

 to the tissue of the lungs. This principle is found in the pulmonary parenchyma at all 

 periods of life, from which it may be extracted by the proper manipulations and obtained 

 in a crystalline form. Its quantity is not very great. The lungs of a female who suffered 

 death by decapitation contained about 0'77 of a grain. 



The action of pneumic acid upon the bicarbonates in the blood has been illustrated 

 in a marked manner by Bernard. When bicarbonate of soda is injected into the jugular 

 of a living animal, a rabbit, for example, it is decomposed as fast as it gets to the lungs, 

 and carbonic acid is evolved. This experiment produces no inconvenience to the animal 

 when the bicarbonate is introduced slowly ; but, when it is injected in large quantity, 

 the evolution of gas in the lungs is so great as to fill the pulmonary structure and even 

 the heart and great vessels, and death is the result. 



Exhalation of Watery Vapor. The fact that the expired air contains a considerable 

 quantity of watery vapor has long been recognized ; and most of the earlier experimenters 

 who directed their attention to the phenomena of respiration made the estimation of the 

 quantity exhaled, and the laws which regulate pulmonary transpiration, the subject of 

 investigation. It is evident that there must be many circumstances materially influencing 

 this process, such as the hygrometric condition of the atmosphere, temperature, extent 

 of respiratory surface, etc., which are of sufficient importance to demand special con- 

 sideration. In many points of view, also, it is interesting to know the absolute quantity 

 of aqueous exhalation from the lungs. 



When the surrounding atmosphere has a temperature below 40 or 43 Fahr., a distinct 

 cloud is produced by the condensation of the vapor of the breath. By breathing upon 

 any polished surface, it is momentarily tarnished by the condensed moisture. Although the 

 fact that watery vapor is contained in the breath is thus easily demonstrated, the estimation 

 of its absolute quantity presents difficulties which were not overcome by the older physi- 

 ologists. With the present improved methods of analysis, however, there are many very 

 accurate means of estimating watery vapor. One method is by the use of Liebig's bulbs 

 filled with sulphuric acid, or tubes filled with chloride of calcium, both of which sub- 

 stances have a great avidity for water. From a large number of observations on his own 

 person and eight others, collecting the water by sulphuric acid, Valentin made the fol- 

 lowing estimate of the weight of water exhaled from the lungs in twenty-four hours : 



In his own person, the exhalation in twenty-four hours was 6,055 grains. 



In a young man of small size, the quantity was 5,042 grains. 



In a student rather above the ordinary height, the quantity was 11,930 grains. 



The mean of his observations gave a daily exhalation of 8,333 grains, or about \\ 

 Ib. av. 



