DR. ANDREWS ON THE GASEOUS STATE OF MATTER. 
427 
to the indications of the air-manometer. Since my former communication in 1869, I 
have given the most careful consideration to this subject, and I hope soon to submit to 
the Society a detailed statement of the only method by which, as far as I can discover, 
this highly important question can be resolved for high pressures. Even for pressures 
under 30 atmospheres the discordant results at which such distinguished physicists as 
Oersted and Swendsen - , Arago and Duloxg, and Regnault have arrived, afford ample 
proof of the extraordinary difficulties of the investigation*. I shall have occasion 
presently to recur to this subject, and to refer to the attempts which have since been 
made by other methods to ascertain the compressibility of air and of the permanent 
gases at high pressures. In the absence of precise data, I have not attempted in this 
paper to correct the indications of the air-manometer so as to reduce them to true 
pressures. We shall see hereafter that up to 250 atmospheres there is good reason for 
believing that the deviations of the air-manometer from true pressures are not consi- 
derable, or such as to interfere with any general conclusions. 
In the present investigation on the properties of the gaseous state of matter I have, 
as in my former researches, selected carbonic acid gas(C0 2 ) for experiment, partly from 
the facility with which it can be prepared in a state of purity, but chiefly from its 
critical temperature being only 31° above the freezing-point of water. It may fairly be 
taken as a typical representative of the gaseous state, and, as we shall see, it is in a 
condition peculiarly favourable for the discovery of the laws which govern the action of 
the internal or molecular forces in that state. The experiments of Oersted and 
Swendsen, of Despretz and of Pouillet, have shown that when exposed to increasing 
pressure it deviates sensibly from the law of Boyle ; and Regnault has measured with 
precision its compressibility at 3° for pressures reaching to 27 atmospheres, and has 
likewise confirmed the observation of Yon Wrede, that even at pressures below one 
atmosphere it deviates sensibly from Boyle’s law. 
The carbonic acid gas was prepared by the action of pure and dilute sulphuric acid, 
deprived of its air by boiling, upon marble. It was carefully desiccated by passing 
through a U-tube filled with fragments of pumice freed from chlorides, and moistened 
with pure sulphuric acid. In Tables I., II., and III. will be found the results of a 
large number of experiments on the compressibility of carbonic acid at temperatures 
differing little respectively from 6°*5, 64°, and 100°. For the lower temperature a stream 
of cold water from a large cistern was made to flow at a uniform rate around the 
carbonic acid tube. The manometer-tube was kept at a steady temperature by a 
similar arrangement. The temperature of 64° was obtained by passing the vapour of 
* Arago and Dtjlojtg inferred from their experiments, extending from 1 to 27 atmospheres, that the law of 
Bovle is strictly true in the case of air ; and the same conclusion was also drawn by Oersted and Swexdsen from 
their own experiments. From a careful analysis of their results, as given in the original memoir of Arago and 
Dtjloxg (Memoires de l’Academie des Sciences, x. p. 207), it appears to me that their experiments rather indicate 
a deviation from the law of Boyle, hut to not more than one third of the amount given by the subsequent 
researches of Regxadlt (Mem. de l’Acad. des Sciences, xxi. p. 421). 
