DR, ANDREWS ON THE GASEOUS STATE OF MATTER. 
449 
to produce a diminution of volume ; the other is due to molecular conditions producing 
a resistance to diminution of volume other than that which occurs in a perfect gas. 
In the case of ordinary liquids the resistance to change of volume from increase of 
pressure is very great ; in such liquids as liquid carbonic acid it is not so great, but it 
augments as the pressure is increased. 
In the case of the permanent gases (or gases which have not hitherto been liquefied) 
the resistance to change of volume becomes predominant between 100 and 200 atmo- 
spheres, and at higher pressure it steadily increases. In hydrogen it appears under the 
lowest pressures at which that gas has been examined. It is probable, therefore, that 
this resistance to change of volume occurs in all gases even at low pressures, although, 
except in the case of hydrogen, it cannot be directly observed at low pressures, from the 
internal attractive forces producing a much greater change in the opposite direction. 
In the play of these opposing internal actions we have the explanation of the singular 
fact that the air-manometer indicates almost true pressures at 200 atmospheres, while 
at lower pressures its indications are above, and at higher pressures below the truth*. 
It is commonly assumed that in hydrogen gas the molecular forces producing internal 
attraction are absent ; but this conclusion is doubtful, as the effects observed are pro- 
bably here also differential. In carbonic acid, on the other hand, the internal attractive 
forces are very powerful, and mask the effects due to resistance to pressure till the gas 
is reduced to a small fraction of its volume under one atmosphere. In the experiments 
from which the laws of the ordinary gaseous state, as enunciated in equations (A), (B), 
and (C), were derived, I have always operated under the most favourable conditions for 
eliminating the influence of resistance to change of volume ; and these laws are therefore 
to be regarded as characteristic of the gaseous state when the properties of the ideal 
gas are modified solely by the action of internal attractive forces. 
* Caiiletet, ‘ Comptes Rendus,’ vol. lxx. p. 1133. 
3 K 
HDCCCLXXVI. 
