J. W. Gibbs — Vapor- Densities. 385 
almost uniformly less than the calculated value. This defi- 
ciency of pressure is doubtless to be accounted for by a fact 
ich MM 
nection. The protochloride of phosphorus deviates quite appre- 
one-half an atmosphere.* Now we may assume as a genera 
rule that when the product of volume and pressure of a gas is 
slightly less than the theoretical number (calculated by the 
laws of Mariotte, Gay-Lussac, and Avogadro) the difference for 
any same temperature is nearly proportional to the pressure,+ 
It is therefore probable that between 160° and 180°, at press- 
: peri 
ments of Wurtz, as exhibited in Table LX, show that the pres- 
sure, and therefore the product of volume and pressure, (we may 
evidently give the volume any constant value as unity,) in a 
mixture consisting principally of the protochloride is on the 
average a little more than two per cent less than is demanded 
y theory, the differences being greater when the proportion of 
the protochloride is greater. The deviation from the calculate 
values is therefore in the same direction and about such in 
. a + 
minishes, The experiments of MM. "roost and Hautefeuille 
show that the bites remarked by M. Wurtz is due to the 
t that on the average in these experiments the deficiency of 
the density of the possible perchloride (compared with the 
= 
Troost and Hautefeuille, Comptes Rendus, vol. lxxxiii (1876), p. 334. 
p. t Andrews, . Oh Os Ganesan Hate ae "Matier.” Phil. Trans., vol. clxvi (1876), 
=~ Geviation of th ide of phosphorus from the laws of ideal gases 
sh . of the protochloride of phosphorus 
Ween dens *nposibility of any very close agreement between such equations as have 
es as occur in the 
org “2@ protochloride alone, it would be rash to attempt to anticipate the 
_ “Suit of experiment, 
