168 
ON THE MOLECULAR MOBILITY OF GASES. 
upon the gaseous condition rather than supersede it. Gay-Lussac made the remarkable 
observation that the vapours emitted by ice and water, both at 0° C., are of exactly 
equal tension. The passage from the liquid to the solid state is not made apparent in 
the volatility of water. The liquid and solid conditions do not appear as the extinction 
or suppression of the gaseous condition, but something stiperadded to that condition. 
The three conditions (or constitutions) probably always coexist in every liquid or solid 
substance, but one predominates over the others. In the general properties of matter 
we have, indeed, to include still further (1) the remarkable loss of elasticity in vapours 
under great pressure, which is distinguished by Mr. Faraday as the Caignard-Latour state, 
or constitution, which intervenes between the liquid and crystalline states, extending 
into both and affecting probably all kinds of solid and liquid matter in a greater or less 
degree. The predominance of a certain physical state in a substance appears to bo a 
distinction of a kind with those distinctions recognized in natural history as being pro¬ 
duced by unequal development. Liquefaction or solidification may therefore not involve 
the suppression of either the atomic or the molecular movement, but only the restriction 
of its range. The hypothesis of atomic movement has been elsewhere assumed, irrespec¬ 
tive of the gaseous condition, and is applied by Dr. Williamson to the elucidation of a 
remarkable class of chemical reactions which have their seat in a mixed liquid. 
Lastly, molecular or diffusive mobility has an obvious bearing upon the communication 
of heat to gases by contact with liquid or solid surfaces. The impact of the gaseous 
molecule upon a surface possessing a different temperature, appears to be the condition 
for the transference of heat, or the heat movement, from one to the other. The more 
rapid the molecular movement of the gas the more frequent the contact, with consequent 
communication of heat. Hence, probably the great cooling power of hydrogen gas as 
compared with air or oxygen. The gases named have the same specific heat for equal 
volumes ; but a hot object placed in hydrogen is really touched 3*8 times more frequently 
than it would be if placed in air, and 4 times more frequently than it would be if placed 
in an atmosphere of oxygen gas. Dalton had already ascribed this peculiarity of hy¬ 
drogen to the high “mobility” of that gas. The same molecular property of hydrogen 
recommends the application of that gas in the air-engine, where the object is to alter¬ 
nately heat and cool a confined volume of gas with rapidity. 
The transpiration-velocity of oxygen being 1, that of chlorine is 1*5 ; that of hydrogen 
2*2G ; of ether vapour at low temperatures the same, or nearly the same, number as 
hydrogen ; of nitrogen and carbonic oxide half the velocity of hydrogen ; of olefiant gas, 
ammonia, and cyanogen 2 (double or nearly double that of oxygen); of carbonic acid 
1*37G, and of the gases of marshes 1*815. In the same gas the transpirability of equal 
volumes increases with density, whether occasioned by cold or pressure. The transpira¬ 
tion-ratios of gas appear to be in constant relation with no other known property of the 
same gases, and they form a class of phenomena remarkably isolated from all else at 
present known of gases. 
There is one property of transpiration immediately bearing upon the penetration of 
the graphite plate by gases. The capillary offers to the passage of gas a resistance ana¬ 
logous to that of friction, proportional to the surface, and consequently increasing as 
the tube or tubes are multiplied in number and diminished in diameter, with the area of 
discharge preserved constant. The resistance to the passage of a liquid through a 
capillary was observed by Poiseuille to be nearly as the fourth power of the diameter of 
the tube. In gases the resistance also rapidly increases; but in what ratio, has not been 
observed. The consequence, however, is certain, that as the diameter of the capillaries 
may be diminished beyond any assignable limit, so the flow may be retarded indefinitely, 
and caused at last to become too small to be sensible. We may therefore have a mass 
of capillaries of which the passages form a large aggregate, but which are individually 
too small to permit a sensible flow of gas under pressure. A porous solid mass may 
possess the same reduced penetrability as the congeries of capillary tubes. Indeed, the 
state of porosity described appears to be more or less closely approached by all loosely 
aggregated mineral masses, such as lime plaster, stucco, chalk, baked clay, non-crystal¬ 
line earthy powders like hydrate of lime or magnesia compacted by pressure, and in the 
highest degree perhaps by artificial graphite. 
3. A plate of artificial graphite, although it appears to be practically impenetrable 
to gas by either of the two modes of passage previously described, is readily penetrated 
by the agency of the molecular or diffusive movement of gases. This appears on com- 
