548 .APPENDIX. 



many hours before that change takes place, by its sudden rise or fall ; 

 but these are omitted for want of room. 



It is known that the quantity of vapor in the air, by weight, may be 

 ascertained at any time by the dew point. So, also, the quantity at any 

 time in the breath, expired from the lungs, may be known by breathing 

 on a bright metallic tumbler of water, and finding the highest tempera- 

 ture at which the vapor will condense upon it. 



I have performed this experiment frequently, both in summer and 

 winter, and I find the dew point of my breath, and that of several 

 others, 94 Q of Fahr. 



Now, by examining a table of the elastic force of vapor or dew points 

 by Dalton, it will be seen that when the dew point is 94^ the elastic 

 force of vapor is 1.53 inches of mercury in the barometer; and as 

 vapor is known to be only five eighths the specific gravity of air, it may 

 easily be calculated that the vapor in the breath is about one thirty-first 

 part of the breath in weight; that is, of thirty-one pounds of the breath 

 expired, if the dew point is 94, thirty pounds will be air, and one pound 

 vapor. This quantity is always much more than that which is inspir- 

 ed ; for the dew point of the atmosphere is never more than 80 g in this 

 climate, and in the winter it is sometimes much below zero. Whatever 

 it may be, it can always be known, and of course the quantity inspired 

 with different dew points can be known. Therefore, if the quantity 

 inspired should be ascertained, and subtracted from the quantity expired, 

 the remainder will be the quantity evaporated from the lungs them- 

 selves. 



According to this principle, I have made the calculation of the 

 quantity of vapor discharged from the lungs, by subtracting the quantity 

 inspired from the whole quantity expired, supposing the dew point of 

 the breath, on expiration, to be constantly 94, as I have always found 

 it to be. 



It results from this calculation, that when the dew point is 32, we 

 evaporate from the lungs one pound of vapor for every thirty-five 

 pounds of air which we breathe. And when the dew point is 75 we 

 evaporate from the lungs one pound for every sixty-nine pounds ; so 

 that in summer, when the dew point is very high, we evaporate from 

 our lungs only about half as much as we do in winter, when the dew 

 point is very low. Moreover, it will appear by calculating according 

 to principles developed in article (9), that the latent caloric contained in 

 one pound of vapor, at the temperature of the breath would be suffi- 

 cient to heat thirty-five pounds of air, about 130, and therefore the air 

 which we breathe at the temperature of 32, brings out with it in the 

 vapor alone, sufficient caloric to heat it 130. And if to this we add 

 the number of degrees it is actually heated, from 32 to 98 Q , it will ap- 

 pear that when we breathe air at 32 Q , the lungs part with a sufficient 

 quantity of caloric to heat all the air we breathe 196. This calculation 

 is made on the supposition that no vapor is generated in the lungs by 

 the union of oxygen and hydrogen. 



M. Pouillet, however, (506,) says that Dulong, in a series of experi- 

 ments, not yet published, has shown that some oxygen disappears in 

 respiration, besides that which unites with carbon to form carbonic acid, 

 and he seems to think it probable that it unites with hydrogen to form 

 water ; if so, it will modify the result given above. 



The whole subject is worthy of further investigation. If it should be 



