( 674 ) 
ence of some factor or other, as the concentration, the temperature, 
on the course of the process. 
MADSEN & Nyman and also Miss CrrcK derive from their obser- 
vations that the influence of the above factors may likewise be 
expressed in formulas. Thus the well-known formula of ARRHENIUS 
in which the relation between temperature and velocity of reaction 
is expressed, would also hold good in this connection. It seems to 
me, for reasons already mentioned, to be prudent not to follow them 
on this path. Therefore we refer, with regard to the points meant, 
to the figures 2 and 3, without commenting on the subject. 
Geology. — “On a long-period Variation in the Height of the 
Ground-water in the Dunes of Holland.” By Prof. Eve. Dusots. 
(Communicated by Dr. J. P. VAN DER STOK). 
Unmistakable and obvious is the lowering that the height of the 
ground-water in the dunes of the provinces of North- and South- 
Holland has undergone in consequence of the lowering of the level 
of the water at their east border (the making dry of the Lake of 
Harlem and of a large part of the IJ) and of deep cuttings in the 
dunes themselves (North-Sea Canal), furtheron, not less, by the 
collecting of large quantities of water supplies for some cities and 
towns. 
From these causes there resulted a lowering which may be called 
a permanent one, inasmuch that soon they have brought about a 
new state of equilibrium with the supply by the part of rainfall 
which soaks in, and the flowing off. This really did take place in 
each case in which certain limits were not transgressed and as long 
as the collecting of water did not increase. 
Side by side with these artificial changes of the height of the 
ground-water in the dunes, there exist also changes by natural, viz. 
climatal causes. These, in this as in other cases, are not continuous, 
but they do occur in periods. Indeed, in the latest historical past, as 
far as data are available, very clearly dry and wet epochs alternate 
with one another. 
The Commission which, in 1891, inquired in the supplying of water 
from the dunes to Amsterdam pointed out, in their report, that from 
1849 till 1856 there was a period of much rain, from 1856 till 
1868 a dry period, again followed by the rainy years of 1869 till 
1882. They showed also (for Utrecht) that under the combined in- 
fluence of rainfall and evaporation such wet and dry epochs are 
