ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 217 



water is sufficient to keep it, except to a comparatively small amount, 

 in the state of a liquid. Yet this small proportion which flows through 

 our atmosphere reaches the enormous weight of 54,460 billions of tons. 

 Lighter than air, transparent, almost impalpable, vapor has an immense 

 work to do in the sustenance of all that grows and breathes upon the 

 surface of the earth. Like a good genius, it enables the air, the sun- 

 shine, the earth, to bring forth their riches, to cover the globe with 

 verdure and gladness, and truly to make the desert blossom like the 

 rose. Without vapor in the air, there would be no streams, no lakes, no 

 wells. The land would be uninhabitable by man, except so far as fresh 

 water might be condensed from sea water by machinery, and plants for 

 his use be grown by the seashore. Even then the human system would 

 hardly tolerate the parching influence of a perfectly dry atmosphere. 



Water vapor, having a low temperature of condensation, was one of 

 the last substances to fall, during the cooling of this globe millions of 

 years ago, from the vaporous into the liquid condition, and consequently 

 remains as a covering between the rocks, which were early solidified, 

 and the air, which was not solidified at all. Water covers about three- 

 fourths 1 of the whole area of the terrestrial ball. It has the remark- 

 able property of being capable of existing in the gaseous, liquid, and 

 solid states within a small range of temperature, and even of existing 

 in all three states under ordinary conditions at temperatures which are 

 common in winter over a large area, and which are easily borne by 

 human beings. 



In every cubic inch of water are many thousands of millions of mil- 

 lions of molecules, and all of these vibrate more or less rapidly under the 

 stroke of heat. Some molecules, as a result of collisions among them- 

 selves, which are very numerous in every second, and as a result of 

 their situation on the surface of the sea, are propelled with such 

 velocity that they leap above the general surface, get beyond the 

 retaining power of cohesion, and are taken up by the wind or by rising 

 currents and carried aloft. The vapor rising from the water surface is 

 warm, has in fact become vapor owing to being in more energetic 

 vibration than the average of the particles of water. Moreover, vapor 

 is lighter than air. So the lowest stratum of vaporous air near the 

 tropical sea becomes lighter than the air above it for three reasons: 

 First, by being in contact with the warm water which has absorbed the 

 sun's rays; secondly, by being mixed with vapor which is lighter than 

 the air it displaces, and thirdly, by this vapor coming from the warmest 

 or most strongly vibrating molecules on the wafer plane. 



The force of gravitation, it should be observed, is often of very little 

 account where small particles such, as these molecules of water are to 

 be considered. A slight charge of electricity would be enormously 

 more powerful in directing the motion of a single molecule. The reason 



1 The proportion of water to laud is about 145,000,000 square miles to 52,000,000 

 square miles. 



