Hygrometry. i 5 
“by a fpontaneous precipitation of water” Ib^ftly, as immerfing 
folid hygrofcopic bodies in water , or expofing them in a medium 
where there is an aCtual precipitation of water (as in a fog) is 
an effe&ual means of furnifhing their pores with the whole quan- 
tity of water they can imbibe ; it is evidently a Jure means of 
producing extreme moifture in them : and this point cannot be 
overpaffed, neither in water , nor in fog, fince it depends upon 
the refiftance of the pores to further dilatation by the mere intro- 
duction of water ; but it muft be attained in an hygrometer , to 
fix its point of extreme moifture . 
23. When formerly I had fixed upon that method for pro- 
curing to my firft hygrometer a true point of extreme moifture , 
it occurred to me, that the temperature of water might in- 
fluence fenfibly the expanfion of its ivory tube ; and in order to 
difcover if it was fo, I made fome experiments,, related in 
§§ 104 &c. of my former paper, the refult of which was, 
that the temperature of water had a fenfible effeCt on the expan- 
fion of ivory . But foon after I diftrufted fome modifications of 
that complicated hygrometer ; and efpecially this particular 
refult. 
24. I then changed that firft method ; which confifted in 
meafuring the changes of capacity of hollow cylinders, into 
that of meafuring the changes in length of hygrofcopic fubftances; 
and for fome preliminary experiments on many of them, I 
made particular frames , in which, by a combination of glafs and 
brafs , the effeCts of heat on thofe materials compenfated each 
other ; by which means the indices of thofe inftruments were 
only affeCted by the modifications of the hygrofcopic fubftances 
tried in them. I have mentioned thefe frames in a paper on 
pyrometry , printed in the Phil. Tranf for 1778. With thefe 
frames I firft tried ivory 9 in water of different temperatures ; 
and 
