( 973 ) 
turning downwards of the convex side (and the point of intleetion caused 
thereby) will be the more pronounced in a miscible substance of the 
group when the so-called “differential heat of imbibition for 7— 0” 
is larger, that is to say, the quantity of heat generated when a large 
quantity of dried substance absorbs one gram of water ®. With 
sulphuric acid this character of the vapour pressure line is more 
strongly pronounced than with phosphoric acid, and with this sub- 
stance considerably stronger than with glycerin. As will be noticed 
from the curve, the behaviour of the imbibing bodies lies more or 
less between that of sulphuric and phosphoric acid. We may, there- 
fore expect as probable that the said heat quantity is greater 
in sulphurie than in phosphorie acid and much larger than in glv- 
cerin, and that in swelling bodies its values lie between those of 
the substances mentioned first. That such is indeed the case is shown 
by the subjoined table for the value of this quantity which I have 
calculated for the different substances from the caloric experiments. 
OER ii ern ts, eee) el ZOO. Cals: sulphuric: acid «… 4.) 550" eal. 
BONDS tet 890”, “orthophorphoric -acid 100° *;, 
maineial starch eranues.. 420 ;, … glyeerin’, nr 20 … 
woody fibres (estimation of 
Bende fibrey) NA see 4.7) 260° |: 
nuclein (from yeast) . . . 200 ,, 
In the aqueous vapour pressure we find therefore also a resem- 
blance, to in many details, between the quantitative laws of swel- 
ling and mixing. 
c. Volume contraction. 
In the case of three amorphous imbibing substances J have deter- 
mined to volume contraction ¢ (in ee) which takes place when 
one gram of dry substance absorbs / grams of water. The method was 
mixtures, for the vapour pressure of mixtures. Particularly in the case of the 
ditatable substances the formulae of Nernsr seem to me to become of great significance. 
They predict for instance, that if the heat of imbibition is represented by a hyper- 
bola, the hygrometric line must have the S-form, and various particulars. of this 
line which are actually observed. I will refer to this more fully shortly in an 
article on the Thermodynamics of the Swelling Process. 
*) The differential heat of mixing is calculated from the ordinary heat of mixing 
dW 
according to the formula w= — 
di 
64 
Proceedings Royal Acad. Amsterdam. Vol. XIII. 
