THE SMITHSONIAN INSTITUTION. 193 



Steam or vapor, at all temperatures, may be considered as water con- 

 taining a definite amount of heat. A pound of steam at 212° contains 

 ver}'- little more caloric than a pound of vapor at 32°. This is proved 

 by the fact that steam, at any temperature, when condensed, is capable 

 of raising the temperature of its own weight of water more than 1,000°. 

 If a cubic foot of steam at 32°, weighing 2.539 grains, were subjected 

 to pressure, and none of the heat evolved during the condensation lost, 

 a small volume of steam at 212° would be produced. But a cubic 

 foot of steam at 32° could not e3^st in the atmosphere by itself, as it 

 would be subjected to a pressure of 30 inches of mercury, whereas it 

 could bear no more than one-fifth of an inch. Hence a difficulty arose 

 amongst chemists in regard to the manner in which water existed in 

 the invisible form in the atmosphere at all temperatures, and under all 

 pressures. There was also some difficulty in accounting for the man- 

 ner in which the mixture of the different gases took place. 



It has been found that air brought down from the tops of the highest 

 mountains, and from the greatest elevation reached by aeronauts, is 

 nearly the same in composition as at the surface of the earth. This 

 fact was rather perplexing, because the different gases of the atmo- 

 sphere have not the same specific gravity. A cubic foot of oxygen 

 weighed more than one of nitrogen, while one of carbonic acid was 50 

 per cent, heavier than either. 



Common air 1.0000 



Carbonic acid 1.5240 



Nitrogen 0.9760 



Oxygen 1. 1026 



Steam ^12° 0.6235 



The carbonic acid being the heaviest gas, it should chiefly occupy 

 the lower stratum of the atmosphere; and the nitrogen the top. Dal- 

 ton, to reconcile a variety of phenomena with each other, added a new 

 proposition to the theory of the atomic constitution of the mixed 

 gases — namely, that the atoms of oxygen do not repel the atoms of 

 nitrogen, 'but only those of their own kind. That the one gas is as a 

 vacuum to the other. On this principle the most complete mixture of 

 gases is explained, the atoms of one gas by their mutual repulsion are 

 forced apart as is in void space, and thus difi;used among those of an- 

 other. At the last lecture we mentioned that if this room were com- 

 pletely exhausted of air, and a cubic inch of hydrogen introduced, it 

 would instantly expand and fill the whole space. Now, the same thing 

 would happen if a cubic inch of hydrogen were introduced into the 

 room filled with air. The process would be much slower, but the 

 mutual repulsion, of the atoms of hydrogen would still be in as active 

 operation as it was in the vacuum, and would still cause them to sepa- 

 rate until they pressed against the walls of the room with as much 

 force as if there were no air in it. 



Not only has the truth of this law been verified, but the diffusive 

 velocity of the various gases ascertained by actual measurement. 

 This velocity is inversely as the density of the gases ; in other words, 

 the lighter the gas the greater its rapidity of diffusion. The vapor of 

 water, or steam, is subject to the same law ; and this is the solution of 

 Mis. Doc. 24 13 



