1 6 PHYSIOLOGICAL CHEMISTRY. 



Per Cent. Per Cent. 



Muscles 75 Brain (gray matter) . . 86 



Spleen 76 Milk 88 



Pancreas 78 Vitreous humor 98.5 



Blood 79 Cerebro-spinal fluid . . 99.0 



Kidney 83 Saliva 99-5 



Air. Besides its content of oxygen, nitrogen and argon 

 the atmosphere contains several other gases in small amount. 

 The most abundant of these is water vapor, with smaller traces 

 of carbon dioxide, helium, neon, etc. As the amount of 

 aqueous vapor present is extremely variable it is customary to 

 give the analysis of the dry air only, which in volume per 

 cent is about this, the rarer gases being included with the 

 nitrogen and the argon : 



Per Cent. 



Nitrogen 78.40 



Oxygen 20.94 



Argon 0.63 



Carbon dioxide 03 



The water vapor present varies with the temperature and 

 other physical conditions and may sometimes make up one 

 per cent, or even more, by weight of the whole mass. A 

 cubic meter of fully saturated air contains 30.1 grams of 

 aqueous vapor at 30 C., and 75 per cent of this is frequently 

 present in the hot, " close " weather of our summers. It is 

 this high proportion of moisture which renders further evap- 

 oration from the skin so difficult, and which therefore contrib- 

 utes greatly to our bodily discomfort. 



The normal carbon dioxide content is given above as 0.03 

 per cent or three cubic centimeters in ten liters. This amount 

 is greatly exceeded in the air of poorly ventilated houses, but 

 is not in itself the cause of the unpleasant sensations expe- 

 rienced in going into such an atmosphere, although this was 

 long believed. It has been found by experiment that one can 

 breathe, although not comfortably, in a pure atmosphere con- 

 taining as much as 3 per cent of carbon dioxide, while an 

 atmosphere contaminated to the extent of i per cent by human 



