EXHALATION OF WATEKY VAPOK. 447 



When the surrounding atmosphere has a temperature below 

 40 or 43 Fahr., a distinct cloud is produced by the condensa- 

 tion of the vapor of the breath. By breathing upon any 

 polished surface, it is momentarily tarnished by the condensed 

 moisture. Though 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 physiologists. Hales collected the vapor of the 

 breath by expiring through wood ashes, 1 which was the first 

 attempt to estimate the amount of this exhalation by absorp- 

 tion. With the present improved methods of analysis there 

 are many very accurate means of estimating watery vapor. 

 One method is by the use of Liebig's bulbs filled with sul- 

 phuric acid, or tubes filled with chloride of calcium, both of 

 which articles 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, Yalentin makes the 

 following estimate of the weight of water exhaled from the 

 lungs in twenty-four hours : 



In his own person, the exhalation in 24 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 quan- 

 tity was 11,930 grains. 



The mean of his " observations gave a daily exhalation of 

 8,333 grains, or about 1-J- Ib. av. a 



1 HALES, Statical Essays, London, 1739, vol. ii., p. 326. Sanctorius, in 

 1614, was the first (MILNE-EDWARDS, Physiologic, vol. ii., p. 602) to attempt the 

 estimation of the exhalation of vapor of water from the body by comparing the 

 gain in weight due to the ingestion of aliments with the loss by transpiration. 

 We pass over the estimates of Lavoisier and Seguin> Keill, Abernethy, and 

 others, and give only the more exact results obtained by Valentin. Dalton, 

 estimating the quantity of air passing through the lungs in respiration, and as- 

 suming that it passes out of the lungs saturated with watery vapor, makes an es- 

 timate of the total exhalation in the twenty-four hours, which corresponds pretty 

 closely with the results obtained by Valentin. 



2 MILNE-EDWARDS, op. dt., p. 621. 



