^ Mr. de Luc on 
capacity for moijlure , receives it but (lowly : as by that firft pro- 
perty it may be taken in lefs quantity ; and by the laft, it will be 
lefs fubjeft to acquire a fenfible quantity of moijlure in the time 
neceffary for the operations. 
jj. pot-ajh and lome other alkaline fubftances afforded the 
firft of thofe properties, but not the laft ; and I had not fixed 
on any fubftance, when, being at Birmingham in the autumn 
of 1782, Mr. James Watt informed me, that his friend Dr. 
Black had found in quicklime a great capacity for moijlure , 
and much flownefs in retaking it : this he knew, by having 
kept a long time the fame lime in a clofe veffel, for drying falts 
and capillary tubes for thermometers. Thefe were the very 
properties I wanted for my purpofe, which thereby I executed 
as foon as 1 came home. I made thofe firft operations in fmall 
glafs veffels, ufing old lime, which I brought again to white 
heat every time I ufed it. Thefe firft trials agreed with my 
theory in its firft point ; that of producing conftantly the fame 
degree of drynefs : as for the fecond, namely, whether that 
degree was extreme , it depended on other experiments. 
4. Being fure then of a fixed degree of drynefs, the number 
of experiments I undertook made me wifh for a means of 
avoiding the frequent repetitions of bringing again my lime to 
white heat ; and having found one which has fucceeded, I am 
going to defcribe the apparatus. The veffel, fig. 1. is of tin, 
3 feet high and 1 in diameter. A glafs plate a, a , a, a , is fixed 
at the top, forming a vertical feet ion of the cylinder at 1 
inch diftance forwards from the axis. A woven brafs-wire 
cage b, b , b, b, is fixed in the veffel through its diameter, in 
order to keep a fpace for the inftruments ; for the fame pur- 
pofe it is open at the top, and alfo oppofite the glafs, where 
the dials of the inftruments are to be feen. For my experi- 
ments, 
