EESPIKATION. 153 



when it goes out from the lungs it will hold fifteen grains. 

 This vapor is invisible, and generally imperceptible. But if 

 the air at 90, containing fifteen grains in a cubic foot, be 

 cooled down to 32, it then can hold only two and a half 

 grains; and the difference between these quantities twelve 

 and a half Drains will be condensed and become visible in 



O 



the form of water. 



350. The cloud of vapor which one seems to expire in a 

 cold day, is caused by this condensation. If, in winter, one 

 or more persons sit in a room sufficiently warm to be com- 

 fortable, the air becomes filled with pulmonary vapor. If 

 the temperature of the room is the same throughout, this 

 vapor is imperceptible to the eye ; but the air near the win- 

 dows, if these are not double, becomes cooled by the action 

 of the outward air, and then this vapor rS condensed, and 

 lodged upon the glass in the form of water. If the air 

 abroad is cooled below freezing point, this condensed vapor 

 freezes upon the windows, and the glass becomes coated 

 with a layer of ice. So we usually find the windows of our 

 sleeping-chambers covered with ice from this cause, in the 

 cold mornings of winter. 



351. The whole of the water thrown off from the lungs in 

 this state of vapor amounts to about seventeen ounces in a 

 day,* which will make five grains and two thirds a minute. 

 This will saturate nearly one half a foot of air at 90 ; but, 

 as air usually contains about one grain of vapor in each 

 cubic foot, it can absorb so much less, and more air will be 

 saturated with the pulmonary vapor. 



352. The insensible perspiration is another and very 

 fruitful source of moisture in the air. The skin is a very 

 active agent, and is incessantly throwing off watery vapor 

 from its surface. When this runs freely in drops, it is called 

 siceat, arid seems to be very abundant. But this is only a 

 small part of the whole of this fluid, which is thrown off 

 through the external surface ; for a much greater quantity 

 is sent off in an invisible form. 



* Valentin and Dalton. 



