446 
because of the facility with which its vapour-tensions can be deter- 
mined with the method to be explained below. 
Dr. Day, Director of the Geophysical Laboratory of the Carnegie 
Institution in Washington, who has great experience of silicates, ad- 
vised me to begin with synthetical quartz and synthetical anorthite 
(Caleiumalumininmsilieate) as adsorbent solids, because these sub- 
stances, when in mass, certainly do not take up water as a solid 
solution and have very little inclination to react chemically with 
water. Dr. Day had the kindness to have both substances prepared 
for me in the most pure condition and to have them powdered in 
a motor-driven agate-mortyr as finely as is possible. The material 
then was sieved through the finest metal sieve (80 meshes pro centi- 
meter). In this way the surface of the solid was made as large as 
possibly can be attained; in this way the best chance was obtained that 
sufficiently large quantities of adsorbed fluid could be observed in 
the case of a solid substance which agrees as well as possible with 
the above requirement. 
In order to get an impression how finely divided the substances 
were, I have suspended a weighed quantity in a known volume of 
water and have determined with the counting-apparatus for blood- 
corpuscles of Thoma, how many microscopically visible particles 
this suspension contained pro m.m*. In this way it was found, that 
1 mgr. of quartz contains 140 million particles and 1 mer. of 
anorthite 120 million. Extraordinarily finely divided powders therefore ! 
I have determined for both substances the amount of water adsor- 
bed as a function of the vapourtension. Nine portions of this powder 
of 1 to 2 gr. each were carefully weighed in crystallizing dishes; 
these were treated as described below, in order to bring them in 
the same condition and then were placed above 9 different mixtures 
of sulphuric acid and water, of which the vapourtension was known. 
There they remained until constant weight. Ignition in a porcelain 
crucible showed the amount of water contained in the material used. 
Controls showed, that the adsorption at the surfaces of the dishes 
was too small, compared with the adsorption at the surface of the 
powder, to have influence of importance. 
Influence of the preliminary treatment of the powder: the 
adsorbed layer consisting of vapour or of jluid. 
Theoretically there exist two possibilities, when a vapour condenses 
on the surface of a solid. Either it is condensed as vapour only, or 
the layer of fluid is formed on the solid. 
