stances such as freshly precipitated silicic acid, or metallie hydroxides 
wherein probably slow chemical changes take place. For instance 
the blue cupric hydroxide when in contact with water gradually 
changes to the black oxide slowly at the temperature of the room, 
rapidly at the boiling heat; the brown ferric hydroxide turns red, 
stannic acid passes into metastannic acid and silicic acid requires an 
increasing amount of potassium hydroxide to redissolve. Fortunately: 
in the case of albuminous substances and polysaccharides (of which 
the biologically important substances mostly consist) such complications 
do not seem to oceur; spontaneous peptonisation, which might oceur 
to the mind, does not proceed (at the temperature of the room, and 
Whilst) possessing a neutral reaction) with appreciable velocity. In 
those cases where | have extended my researches to metallic hydroxides 
ete. | have provisionally, confined myself to substances which by 
ageing artificially — prolonged heating under water — had been 
freed as far as possible from this complication. To make quite sure, 
the samples of the other substances tested were at least one year old.) 
a. Heat of imbibition and heat of mixing. 
The quantity of heat (IW) was determined (in gram calories) gene- 
rated when one gram of dried substance ?) takes up 7 gram of water; 
this quantity T will call the imbibition heat at the imbibition degree 7*). 
The substance was put into a glass tube fitted with an india rubber 
stopper which was then piaced in a calorimeter filled with water 
until there was an equilibrium of heat. By smashing the bottom of the 
1) All the substances were investigated in the powdered condition; check ex- 
periments had shown that the water adsorption at the surface of the grains of 
the powder was so small that no hindrance to speak of occurred. 
*) The name “dry substance” is somewhat vague. [ chose as such the substance 
dried at 110° in vacuo over sulphuric acid. In this manner all the different sub- 
stances have been obtained as much as possible in a comparable condition. 
The substances with the lowest water content with which | have carried out 
measurements were those dried over sulphuric acid at the temperature of the 
room; the values for the “dry” substance were obtained by extrapolation. In the 
case ol a few substances which cannot be heated to 110° without decomposition 
| have determined the water by drying in vacuo over sulphuric acid at the tempe- 
rature of the room; this is then mentioned separately. Ropewarp and his pupils 
Karrem and VorBenr applied this method of water determination to all substances 
they investigated. The results of the two methods usually differ 1/; to 1° 9. 
5) The imbibition degree ¢ therefore is the number of grams of water d absorbed 
by one gram of dry substance. 
