VAPOR-DENSITIES. 383 



to this source may evidently be diminished by diminishing the 

 intervals of temperature between the successive experiments in a 

 series of this kind, and also by diminishing the opening made in 

 the flask, which increases the time for which the flask may be left 

 open without danger of the entrance of air. In the III d series of 

 experiments by Deville and Troost, the intervals of temperature did 

 not exceed ten degrees (except after the density had nearly reached 

 its limiting value), and the necit of the flask was drawn out into a 

 very fine tube. 



In Table II, which relates to experiments on the same substance 

 at pressures less than that of the atmosphere, the principal series 

 is that of Naumann,* which commences a few degrees below the 

 lowest temperatures of Deville and Troost, and extends to 6 

 centigrade, the pressures varying from 301 to 84 millimeters. These 

 experiments were made by the method of Gay-Lussac. The numbers 

 in the column of observed densities have been re-calculated from 

 the more immediate results of the experiments, and are not in all 

 cases identical with those given in Professor Naumann's paper. 

 Every case of difference is marked with brackets. Instead of the 

 numbers [2'66], [2'62], [2*85], [2*94], Naumann's paper has 2'57, 2*65, 

 2*84, 3*01, respectively. In some cases the temperatures and pressures 

 of two experiments are so nearly the same that it would be allowable 

 to average the results, at least in the column of excess of observed 

 density. In such cases the numbers in this column have been 

 united by a brace. The greatest difference between the observed 

 and calculated densities is *16, which occurs at the least pressure, 

 84 millimeters. In this experiment the weight of the substance 

 employed is also less than in any other experiment. Under such 

 circumstances, the liability to error is of course greatly increased. 

 The average difference between the observed and calculated densities 

 is '063. Since these differences are almost uniformly positive and 

 increase as the temperature diminishes, it is evident that they might 

 be considerably diminished by slight changes in the constants of 

 equation (10), without seriously impairing the agreement of that 

 equation with the experiments of Deville and Troost. But it has 

 not seemed necessary to re-calculate the formula, which, in its present 

 form, will at least illustrate the degree of accuracy with which 

 densities at low pressures and at temperatures below the boiling 

 point of the liquid may be derived from experiments at atmospheric 

 pressure above the boiling point. Moreover, the excess of observed 

 density may be due in part to a circumstance mentioned by Professor 

 Naumann, that the chemical action between the vapor and the 



Berichte der deutschen chemischen Gesellschaft, Jahrgang xi (1878), S. 2045. 



