J. W. Gibbs — Vapor- Densities. 377 
They may be thought to corroborate slightly the tendency 
observed in the experiments of Naumann a roost toward 
rature and pressure, gives a trifling excess of observed 
this single temperature, as having only two constants, of whi 
one is determined so as to make the formula give the theo- 
retical value for infinitesimal pressures, and the other so as to 
make it agree with the experiments of Cahours at the pressure 
of one atmosphere. 
An entirely different method has been employed by Horst- 
Maan* to determine the vapor-density of this substance. A cur- 
rent of dried air is forced through the liquid acid, which is heated 
to promote evaporation, and the mixture of air and vapor is 
chemischen Gesellschaft, Jahrg. iii (1870), S. 78’ 
* Berichte der deutschen 
and Jahrg. xi (1878), $. 1287 
