( 971.) 
bh. Water parpour Pressure. 
In the case cf forty swelling substances, | have determined the 
curve according to which the aqueous vapour pressure of the swollen 
body depends on the degree of imbibition. That pressure has been 
expressed as fraction (h) of the maximum pressure of water vapour 
at the same temperature. *) Such a line which is characteristic for 
the manner in which the imbibition water is contained in a substance, 
1 will call the hygrometric line of the swelling substance. The deter- 
minations were carried out according to a method whieh agrees in 
the’ main with the one employed by vaN BEMMELEN *). 
Of eight of these substances, the lines, again drawn carefully to 
scale, are shown in the illustration. *) They are casei, cellulose, gelatin 
(the best commercial, after being thoroughly washed with water), peptone 
(amphopeptone prepared by Dr. G. GrÜBELER according to KtuNE), gem 
arabic (finest commercial, in powder), seruma/bummn (Albumin aus Blut 
puriss. Merck, dialysed, filtered and then evaporated at the tempe- 
rature of the room), ¢ricaleium phosphate (calcium phosphoricum 
tribasteum sieeum Merck) and artificially aged silicic acid (acumid 
silicum Merck, heated for half a vear under water at 80°C.). The 
first three and the last two have a limited imbibition power, the 
fourth, fifth, and sixth an unlimited one; some of these substances 
belong to the albuminous bodies, others to the polysaccharides, others 
again to the inorganic compounds. Although substances of a very 
different nature have, therefore, been used, all lines appeared to 
possess the same form‘). With small degrees of imbibition the curve 
‘) This quotient alters but very little on the ri-e or fall of the temperature; it 
is, therefore, advantageous to express the vapour pressure in that manner, 
*) The difference with VAN BemMMELEN’s method chiefly consisted in the fact that 
a same portion of the substance did not pass successively through the different 
equilibria but that different portions of a same substance, after the same prelimi- 
nary treatment (maximum drying or drenching), came each simultaneously in equi- 
librium over another sulphuric acid solution This considerably shortens the time 
of the experiments otherwise so tedious. 
3) A double arrow | on the curve indicates that the equilibrium was approached 
from two sides, a single arrow Î or ¥ that it was reached from one side only 
and from which one. Check experiments with numerous substances have shown 
that the form of the lines obtained is the same whether the one or the other 
method is followed and that the equilibria reached differ but little quantitatively. 
') Apart, of course, from this difference that the curve for == 1 terminates 
with substances of limited imbibition power in the imbibition maximum, whereas 
with those of unlimited imbibition power, it takes an asymptotic course. But for the 
rest the lines of these two groups exhibit no difference. 
