“Yes, doubtless, gentlemen,” she continued, “the water will grow less, and, perhaps, in the end totally disappear; yet, it is not this lack of water which in itself will bring about the end of things, but its climatic consequences. The decrease in the amount of aqueous vapor in the atmosphere will lead to a general lowering of the temperature, and humanity will perish _with cold_. “I need inform no one here that the atmosphere we breathe is composed of seventy-nine per cent. of nitrogen and twenty per cent. of oxygen, and that of the remaining one per cent. about one-half is aqueous vapor and three ten-thousandths is carbonic acid, the remainder being ozone, or electrified oxygen, ammonia, hydrogen and a few other gases, in exceedingly small quantities. Nitrogen and oxygen, then, form ninety-nine per cent. of the atmosphere, and the vapor of water one-half the remainder. “But, gentlemen, from the point of view of vegetable and animal life, this half of one per cent. of aqueous vapor is of supreme importance, and so far as temperature and climate are concerned, I do not hesitate to assert that it is more essential than all the rest of the atmosphere. “The heat waves, coming from the sun to the earth, which warm the soil and are thence returned and scattered through the atmosphere into space, in their passage through the air meet with the oxygen and nitrogen atoms and with the molecules of aqueous vapor. These molecules are so thinly scattered (for they occupy but the hundredth part of the space occupied by the others), that one might infer that the retention of any heat whatever is due rather to the nitrogen and oxygen than to the aqueous vapor. Indeed, if we consider the atoms alone, we find two hundred oxygen and nitrogen atoms for one of aqueous vapor. Well, this one atom has eighty times more energy, more effective power to retain radiant heat, than the two hundred others; consequently, a molecule of the vapor of water is 16,000 times more effective than a molecule of dry air, in absorbing and in radiating heat—for these two properties are reciprocally proportional.