A statement from Graham - Journal of Chemical Education (ACS

A statement taken from Thomas Graham's "Elements of Chemistry" (1843) on the diffusion of gases. Keywords (Audience):. General Public. Keywords (Domai...
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A Statement from Graham . . . I n entering into this inquiry I found, first, that gases diffuse into the atmosphere and into each other, with different degrees of ease and rapidity. This was observed by allowing each gas to diffuse from a bottle into the air through s. narrow tube, taking care, when the gas was lighter than air, that it was allowed to escape from the lower part of the vessel, and when heavier from the upper part, so that it had, on no occasion, any disposition to flow out, but wss constrained to diffuse in opposition to the effect of gravity. The result was, that the same quantity of different gases escapes in time which w e exceedingly unequal, but have a relation to the specific gravity of the gaq. The light gases diffuse or escape most rspidly; thus, hydrogen escapes five times quicker than carbonic acid, which is twenty-two times heavier than that gas. Secondly, in the case of an intimate mixture of two gases, the most diffusive gas separatesfmm the other, and leaves the receiver in the greatest proportion. . . I t can be shown, on the principles of pneumatics, that gases should rush into a vacuum with velocities eorresponding to the numbers which have been found to express their diffusion volumes; that is, with velocities inversely proportional to the square root of the densities of thegases. Thelsw of the diffusion of gases has an this account been viewed by my friend, Mr. T. S. Thompson, of Clitheroe, as a conformation of Dr. Dalton's theory; that gases are inelastic towards each other.' I t most be admitted that the ultimate result in diffusion is in strict accordance with Ihlton's law, but there are certain eireumstmces which make me hesitate in adopting it as a true representation of t,he phenomenon, although it affords a convenient mode of expressing it. 1. It is supposed, on that law, that when a cubic foot of hydrogen gas is allowed to communicate with a cubic foot of air, the hydrogen ezpands into the space occopied by the sir, as it would do into a. vacuum, and becomes two cubic feet of hydrogen of half density. The air, on the other hand, expands in the same manner into the space occupied by the hydrogen, so as to became two cubic feet of air of half density. Now if the gases actually expanded through each other in this manner, cold should be produced, and the temperature of the mixed gases should fall 40 or 45 degrees. But not the slightest change of temperature occurs in diffusion, however rapidly the process is conducted. 2. Although the ultimate result of diffusion is always in conformity with Dalton's law, yet the diffusive process takes place in different gases with very differentdegrees of rapidity. Thus, the external air penetrates into a. diffusion tuhe with velocities denoted by the following numbers, 1277, 623,302, accordingly as the diffusion tube is filled with hydrogen, with carbonic acid, or with chlorine gas. Now, if the air were rushing into a vacuum in all these cases, why should i t not always enter it with the same velocity? Something more, therefore, must be msumed than that gmes are vacua to each other, in order to explain the whale phenomena observed in diffusion.

* Phil. Mag., 3rd. series, IV,

321. taken from Thomas Graham's "Elements of Chemistry," published hy Lea and Blanchard, Philadelphia, 1843.



Journal of Chemicol Education