354 RELATIVE ATTRACTIONS OF 
per centages of water in the crystals of some soda 
salts, by submitting them to the drying agency of 
a vacuum accompanied by a vessel of sulphuric 
acid, the results of which were communicated to - 
this Society, I frequently felt at a loss to know 
whether the acid I had under the receiver was 
sufficiently strong to render the space perfectly 
anhydrous ;_ or, indeed, to render it sufficiently 
dry to deprive the salts under operation of as 
much water as anhydrous space could do; and, 
consequently, I much more frequently renewed 
the acid than was really requisite. The annoy- 
ance of doubt, thus frequently felt in impairing 
that spirit of confidence which always ought to 
accompany philosophical investigation, proved 
itself a stimulus to subsequent experimental in- 
quiry, the result whereof furnishes us with facts 
on which we may, I hope, rely in after research; 
and adds, though little, in assisting to fill up 
the vast hiatus remaining to be filled up before 
our knowledge of Nature’s laws can be said to 
be complete. 
The object of my inquiry was, to determine at 
what degree of concentration the affinity of sul- 
phuric acid for aqueous vapour is equal to that of 
anhydrous space for the same vapour at particu- 
