experiments. For those executed up to the present, are too few in 
number to serve as the basis of a fruitful discussion as to the value 
of the different theories. 
In the first place then we want the determination of the line 
according to which the water vapour pressure of a swollen crystal 
depends on the degree of imbibition. 
In a number of imbibing crystals, therefore, I have determined 
the form of this line. The water vapour pressure was expressed as 
a fraction (4) of the maximum pressure of water at the same tem- 
perature, the degree of imbibition as the number of grams of water 
(7) absorbed by one gram of dry substance (hygrometric line). 
In the first place I have made experiments with pure carbon 
monoxide-haemoglobin from horse’s blood and from dog’s blood, which 
I prepared according to the method described by PEKELHARING*), in 
his laboratory and under his guidance. The substance investigated 
consisting of small beautiful rhombic crystals was pressed dry between 
two plates of unglazed porcelain and then placed in small weighed 
glass dishes over sulphuric acid-water mixtures of known vapour 
pressure until an equilibrium was attained; care being taken that 
the small glass bell-jars in which these experiments were carried 
out, contained sufficient carbon monoxide to prevent dissociation of 
the compound. The substance now in equilibrium was, after a 
spectroscopic test, found to consist of pure carbon monoxide- 
haemoglobin and not a trace of methaemoglobin could be detected ’). 
In each of these portions of the compound the water content was 
determined by drying in an airbath at 115°. The experiment was 
made once with carbon monoxide-haemoglobin from dog’s blood 
and twice with that from horse’s blood. The results of these last 
two experimental series agreed very well mutually, a new indication 
that pure haemoglobin has a composition of relatively great constancy. 
The hygrometric lines obtained have been reproduced in the 
illustration. They exhibit a continuous course, quite different from 
that of the salts containing water of crystallisation. In the latter, the 
vapour pressure, wien the water content increases, remains constant 
until the salt has been completely converted into hydrate; if then 
the substance takes up still more water, another hydrate begins to 
1) Voordrachten over Weefselleer, p. 271. 
2) Analogous experiments were first made with oxyhaemoglobin but did not lead 
to results of full worth, because during the forming of the equilibrium a portion 
of this substance was converted into its isomer methaemoglobin. Prof. PEKELHARING 
then advised me to continue the experiments with carbon monoxide-haemoglobin. 
