632 REGNAULT’S HYGROMETRICAL RESEARCHES. 
meters 15 and 16 exhibited the same coincidence with one an- 
other; the hygrometer No. 12 constantly indicated lower num- 
bers. The greatest differences between the indications of these 
five hygrometers amounted to 5°. 
These observations show that it is impossible to calculate a 
single table which applies exactly to all hygrometers, and it is 
desirable that observers should have at their disposal a sim- 
ple process which permits them to construct the table of their 
hygrometer themselves, and by which they may verify the gra- 
duations of their instrument as often as they desire. The pro- 
cess which I shall describe appears to me to satisfy these con- 
ditions perfectly. 
I prepared mixtures of sulphuric acid and water in definite 
proportions, so as to produce the following hydrates :— 
SO? + 2H20, SO? + 3H20, SO?+ 4H20, 
SO? + 5H20, SO*+ 6H20, SO?+ sH20, 
SO? + 10H20, SO*+12H20, SO + 18H20. 
These mixtures were verified by a strict chemical analysis, 
and their composition was rectified whenever analysis showed 
that it differed perceptibly from the composition desired. I 
determined with the greatest care the elastic forces of the aque- 
ous vapour given by these solutions for temperatures comprised 
between 0° and 50° C., employing the process described at page 
569 (memoir on the tensions of aqueous vapour). I constructed 
graphically the curves given by these experiments, and by means 
of three equidistant determinations I ascertained the three con- 
stants which enter into the formula f= a, + a, &. 
I obtained in this manner a formula of interpolation for each 
solution of sulphuric acid. 
I constructed by means of these formulz a general Table, 
which contains the tensions of aqueous vapour given by these 
different mixtures of sulphuric acid and water for each degree 
of the centigrade thermometer, from +5° up to +35° By the 
side of the tensions of each solution I have placed, in a contigu- 
ous column, the relations of these tensions to those given by 
pure water at the same temperature; in other words, the frac- 
tions of saturation produced by these solutions. 
