KONINKLIJKE AKADEMIE VAN WETENSCHAPPEN 
TE AMSTERDAM. 
PROCEEDINGS 
VOLUME XxXIll 
No.6. 
President: Prof. H. A. LORENTZ. 
Secretary: Prof. P. ZEEMAN. 
(Transiated from: ‘Verslag van de gewone vergaderingen der Wis- en 
Natuurkundige Afdeeling," Vols. XXVIII and XXIX). 
CONTENTS. 
G. KRUTKOW: “On the determination of quanta-conditions by means of adiabatic invariants”. 
(Communicated by Prof. P. EHRENFEST), p. 826. 
H. ZWAARDEMAKER: “On Sensibilization to Radioactivity by the Action of Hormones”, p. 838. 
M. W. WOERDEMAN: “An Inquiry into the Distribution of Potassium-compounds in the Electric 
Organ of the Thorn-back (Raja Clavata)”. (Communicated by Prof. H. ZWAARDEMAKER), p. 842. 
(With one plate). ; 
ALPH. DEUMENS: “Extinction by a Blackened Photographic Plate as Function of Wavelength, 
Quantity of Silver, and Size of the Grains’. (Communicated by Prof. W. H. JULIUS), p. 848, 
W. DE SITTER: “On the possibility of statistical equilibrium of the universe”, p. 866. 
J. K. A. WERTHEIM SALOMONSON: “The Mechanism of the Automatic Current Interrupter”, p. 869. 
J. F. VAN BEMMELEN: “The wing-design of mimetic butterflies”, p. 877. 
J. J. VAN LAAR: “On the Equation of State for Arbitrary Temperatures and Volumes. Analogy 
with Planck’s Formula” Il. (Communicated by Prof. H. A. LORENTZ), p. 887. 
R. MAGNUS and A. DE KLEYN: “The function of the Otolithes”, p. 907. (With two plates). 
B. P. HAALMEIJER: “On elementary surfaces of the third order”. (Fifth communication). (Communi- 
cated by Prof. HENDRIK DE VRIES), p. 920. 
W. H. KEESOM: “The quadrupole moments of the oxygen and nitrogen molecules”. (Communicated 
by Prof. H. KAMERLINGH ONNES), p. 939. 
W. H. KEESOM: “The cohesion forces in the theory of VAN DER WAALS”. (Communicated by Prof. 
H. KAMERLINGH ONNES), p. 943. 
L. E. J. BROUWER: “Intuitionistische Mengenlehre”, p. 949. 
L. E. J. BROUWER: “Besitzt jede reelle Zahl eine Dezimalbruchentwickelung ?” p. 955. 
Erratum, p. 964. 
54 
Proceedings Royal Acad. Amsterdam. Vol. XXIII 
