on Evaporation . 415 
suits, which I gave in my last paper, have confirmed my 
doubts, on even the acquisitions of weight being propor- 
tional to the increase of moisture in the medium ; since they 
do not keep the same pace in different substances. 
The other means was, to introduce in a dry vessel succes- 
sive equal quantities of water without opening the vessel, 
and to observe their effect on the hygroscope. I made, last 
year, a first attempt of that experiment, which succeeded in 
respect of the introduction of water in a space of a known 
small degree of moisture ; but the event confirmed also the 
uncertainty that I suspected in that method, because of a va- 
riable share of water retained by the vessel itself. However, 
as some parts of my apparatus might contribute to that un- 
certainty, I intend to correct it, and repeat the experiment ; 
which, if I obtain regular results, I shall have the honour to 
communicate to the Royal Society. 
I have now summed up the series of propositions which 
connect together in one system the whole of the fundamental 
phaenomena of hygrology and hygrometry ; and the only 
part of that system which remained to be proved by imme- 
diate experiments is, that link mentioned above between the 
two classes of phaenomena, namely ; “ That in vacuo , as in air, 
“ the product of evaporation affects the hygroscope as it 
“ does the manometer.” I had ascertained that fact since 
the year 177 6, by immediate experiments ; which was the 
reason why, in my last work, I considered that circumstance 
as a principle, which, at least, could not be denied by any 
thing known to the contrary. But, for an immediate demon- 
stration of it, many conditions were wanting in my first ex- 
periment, which could not be obtained but by the whole of 
3 H 2 
