OF GASES AT HIGH EXHAUSTIONS. 
431 
when working near the level of the sea. In diagrams A, B, and C I have started 
from this pressure of 760 millims., and have given the curves through a wide range of 
exhaustion. But the curves might also be continued, working downwards instead of 
upwards. Although it is unsafe to indulge in speculation in the absence of data, there 
are some conclusions which it is quite legitimate to draw from an inspection of the 
curves. From the shape and direction in which they cut the 760 line it is reasonable to 
infer their further progress downwards, and we may assume that an easily liquefiable gas 
will show a more rapid increase in log. decrement than one which is difficult to liquefy 
by pressure. For instance, hydrogen, the least condensible of all gases, shows no 
tendency to increase in log. decrement by pressure. Oxygen and nitrogen, which are 
only a little less difficult to condense than hydrogen, show a slight increase in log. 
decrement. Carbonic anhydride, which liquefies at a pressure of 56 atmospheres at 
15° C., increases so rapidly in viscosity that at this pressure it would have a loga¬ 
rithmic decrement of about l - 3, representing an amount of resistance to motion that 
it is difficult to conceive anything of the nature of gas being capable of exerting. 
Kerosoline vapour is rendered liquid by pressure much more readily than carbonic 
anhydride. Its curve of viscosity on diagram A shows a great increase in density for 
a very slight access of pressure (705). 
709. Maxwell’s law was discovered as the consequence of a mathematical theory. 
It presupposes the existence of gas in a “ perfect” state—a state practically unknown 
to physicists, although hydrogen gas very nearly approaches that state. An ordinary 
gas may be said to be bounded, as regards its physical state, on the one side by the 
sub-gaseous or liquid condition, and on the other side by the ultra-gaseous condition. 
A gas assumes the former state when condensed by pressure or cold, and it changes to 
the latter state when highly rarefied. Before actually assuming either of these states 
there is a kind of foreshadowing of change, with partial loss of gaseity. When the 
molecules, by pressure or cold, are made to approach each other more closely, they 
begin to enter the sphere of each other’s attraction, and therefore the amount of 
pressure or cold necessary to produce a certain density or viscosity is less than the 
theoretical amount by the internal attraction exerted on each other by the molecules. 
The nearer the gas approaches the point of liquefaction the greater is the attraction of 
one molecule to another, and the amount of pressure required to produce any given 
density will be proportionally less than that theoretically required by a “ perfect ” gas. 
This foreshadowing of the sub-gaseous or liquid state explains how it is that there is 
such wide divergence from Maxwell’s law in the case of imperfect gases, such as 
carbonic anhydride, water gas, and the volatile hydrocarbon kerosoline. At the other 
end of the scale we find even a more marked divergence from Maxwell’s law. This 
is due to the gas commencing to assume ultra-gaseous properties. 
