290 an W. Gibbs — Vapor-Densities. 
‘01 in the density in experiments at atmospheric pressure, 
would make a difference of ‘21 or ‘47 in the circumstances of 
. In fact, the numbers obtained differ con- 
siderably from those demanded by the formula. 
There remain four experiments by Playfair and Wanklyn* in 
which Dumas’ method was varied by diluting the vapor with 
nitrogen. The numbers in the column of pressures represent 
the total pressure diminished by the pressure which the nitro- 
gen alone would have exerted. They are not quite accurate, 
since the data given to the memoir cited only enable us to 
experiment, equal to760™". The effect of this inaccuracy upo 
the calculated densities would be small. Two of these obser- 
vations agree closely with the formula; and two show considera 
ble divergence, but in opposite directions, and these are the two 
in which the quantities of peroxide of nitrogen were the small- 
est. The differences appear to be attributable rather to the 
difficulty of a precise determination of the quantities of nitro- 
gen and of vapor, than to any effect of the one upon the other. 
Special interest attaches to experiments at the same or nearly 
the same temperature but different pressures. For with exper 
iments at the same temperature, the constants of the formwa 
which are determined by observation are reduced to one, 80 
that the verification of the formula by experiment cannot poe 
sibly be regarded as a case of interpolation. It is not nect 
sary that the temperatures should be exactly the same, for it 
will be conceded that the formula represents the actual func: 
tion well enough to answer for adjusting slight differences 0 
temperature; but itis necessary that the range of p : 
should be considerable in order that the differences of density 
should be large in proportion to the probable errors of observ 
tion, But the pressures must not be so low that accurate 
as is evidently allowable, the observed values follow a 
closely the fluctuations of the calculated, extending from 22 
* Trans. Roy. Soe, Edinb., vol. xxii (1861), p. 463. 
