concern iii,^' llic qiiantilics of wntcr \vliicli fioiii ilir laiiifall poiictralos 

 the soil, it need not be further demonstrated that in tlie f^nb-soil under 

 the dnnes, nndcr the adjacent flat e]e\ate(l area and nnder some 

 shallow polders, drinkalde water is and will not i)e lacking, in the 

 main land of the pro\inces Norlli- and Soulli-lIoUaiid, superficiallj' 

 judging so little favoured in this respect, and with two fifths of the 

 population of our country. That the velocity with which the deep 

 uiulerground water can move through the coarse-grained sand, is quite 

 sufficient to make it possible to procure it from the sand in large 

 quantities, a great number of facts prove it. I will mention but one, 

 that of the paper-manutactory at Velsen, of which the six wells, 

 encompassing an area of 0.85 H.A., every 24 hours, on the average, 

 furnish at least 2200 M" of fresh water or nearly as nnu-h as the 

 town of Harlem wants and about a tenth of Avhat Amsterdam 

 consumed during these latter years. And those wells funiish water, 

 which shows no signs as yet of a too slow horizontal motion ere 

 long being likely, by disturbing the natural ecpulibrium of the under- 

 ground fresh and salt-water, to convert the pumped fresh into salt- 

 water. On the contrary the water of the oldest well, full six years 

 in use, has grown a little sweeter still. 



Physics. — "Lwestlgt/tidii of 'i .source of oi'ors in measiirempnfs 

 of magnetic rotations of the plane of pi'liirisalion in atisoi-hnuj 

 solutions." By Dr. L. H. Sh'-.rtsema. (Communication N". 91 

 from the Physical Laboratory of Leiden by Prof. H. Kameulingii 

 Onnes.) 



(Communicated in the meeting of January 30, 1904). 



In a great number of measurements of the magnetic rotation of 

 the plane of polarisation it was found, that this rotation assumes very 

 large values in the neighbourhood of an absorption-band. Similar large 

 values were found by me in an investigation on the negative mag- 

 netic rotation of potassium ferricyanide ^) in dilute solutions. Tiiese 

 results ao-ree with the new optical theories which \ ield for the 

 magnetic rotation the dispersion formula: 'j 



e A d7i 



^ ~m2VT). 



dn 

 since the ouautitv — also assumes a large value near a i)and. 



1) Arch. Need. (2) ö p. iiT: These Proc. 1901 '02 p. 339; Conim. Pliy>. Lab. 

 Leiden N". 02, 76. 



2) These Proc. 1902/03 p. 413; Comm. Phys. Lab. N'l 82. 



