70 VAPORIZATION. 



the lightest uppermost, and the heaviest undermost, as in the 

 familiar example of oil and water, unless they have combined 

 together. This peculiar property of gases has repeatedly been 

 made the subject of careful experiment. Common air, for 

 instance, is essentially a mixture of two gases, differing in 

 weight in the proportion of 9/6 to 1103, but the air in a tall 

 close tube of glass several feet in length, kept upright in a 

 still place, has been found sensibly the same in composition 

 at the top and bottom of the tube, after a lapse of months. 

 Hence, there is no reason to imagine that the upper strata of the 

 air differ in composition from the lower ; or that a light gas, such 

 as hydrogen, escaping into the atmosphere will rise, and ulti- 

 mately possess the higher regions; suppositions which have 

 been made the groundwork of meteorological theories at dif- 

 ferent times. 



The earliest observations we possess on this subject are 

 those of Dr. Priestley, to whom pneumatic chemistry stands 

 so much indebted. Having repeated occasion to transmit a gas 

 through stoneware tubes surrounded by burning fuel, he per- 

 ceived that the tubes were porous, and that the gas escaped out- 

 wards into the fire, while at the same time the gases of the 

 fire penetrated into the tube, although the gas within the tube 

 was in a compressed state. 



Dr. Dalton, however, first perceived the important bearings 

 of this property of aerial bodies, and made it the subject of 

 experimental inquiry. He discovered that any two gases, al- 

 lowed to communicate with each other, exhibit a positive ten- 

 dency to mix or to penetrate through each other, even in 

 L opposition to the influence of their weight. Thus, a 

 vessel containing a light gas (hydrogen,) being placed 

 above a vessel containing a heavy gas (carbonic acid,) 

 and the two gases allowed to communicate by a narrow 

 tube, as represented in the figure, an interchange speed- 

 ily took place of a portion of their contents, which it 

 might have been supposed that their relative positi'on 

 would have prevented. Contrary to the solicitation of 

 gravity, the heavy gas continued spontaneously to ascend 

 and the light gas to descend, till in a few hours they 

 became perfectly mixed, and the proportion of the two 

 gases was the same in the upper and lower vessels. 

 This disposition of different gases to intermix, appeared 

 to Dr. Dalton so decided and strong, as to justify 



