216 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



mingle with the air in the room, but carried off by the chimney, as with 

 coal fires. The poisonous gases, etc., generated by combustion are 

 very apt to cause sore throats, headache, and other ailments, and may 

 favor the incidence of diphtheria. Carbon monoxide, which is given 

 off by charcoal, coke, and gas fires in small quantities, is a strong 

 poison. 



VAPOR OF WATER. 



The atmosphere of vapor of water coexisting with and interpene- 

 trating the atmosphere of nitrogen and oxygen is of no less impor- 

 tance to human life. Its physical properties are very different and its 

 characteristic is variety of state, while that of the dry air in which it 

 floats is uniformity of state. Air is solid at —328° F. under a pressure 

 of 1,000 atmospheres; vapor of water is solid at 32° F. under a pres- 

 sure of 1 atmosphere. Eecent researches have proved that cohesion, 

 the force by which bodies are held together, increases as temperature 

 is reduced. At the exceedingly low temperature of 328° F. metals 

 and other solids are firmer than at any higher degree. Heat is there- 

 fore a force by which the molecules of substances in general are driven 

 further asunder in the whole range of temperature. The force of cohe- 

 sion is less in gases than in liquids and solids; and, indeed, is not 

 manifested at all at ordinary temperatures and pressures. By great 

 cold and great pressure, however, all gases but one have been brought 

 to the liquid condition, wherein cohesion obtains the advantage over 

 heat, and it is almost certain that by still greater cold all gases would 

 be enabled to exist as cohesive solids. The habitable state of our globe 

 depends on the adjustment of temperature and atmospheric density so 

 as to permit the elements of life to maintain their appropriate gaseous 

 ami liquid forms. It is the large diversity of melting and boiling 

 points in different substances which makes life possible. Uniformity, 

 or even an approximation to it, would be fatal. 



Water vapor, instead of being nearly homogeneous and of equal den- 

 sity at equal heights above the earth, varies greatly in quantity at dif- 

 ferent times and in different places. Like a gas, it tends to diffuse 

 itself uniformly through the atmosphere as in a vacuum, but the resist- 

 ance of the air has the effect of retarding the rate of diffusion. Owing, 

 moreover, to the never-ceasing operation of unequal condensation and 

 evaporation, the distribution of vapor is very unequal, both in time 

 and place. The average quantity near the sea level in most countries 

 is from 60 to 75 per cent of that required for complete saturation. 



While air is always a mixture of gases in a fixed proportion, very far 

 beyond any possible cause of liquefaction or solidification, vapor is 

 never far from its condensing point; that is, however high the tempera- 

 ture and however low the pressure, a moderate amount of cooling will 

 always bring it into the condition of water or ice. The repulsive force 

 in the perfect gas, or in air, is sufficient to keep it gaseous at the lowest 

 conceivable temperatures in natural conditions; the cohesive force in 



