1247 
validity of that term has been proved with still greater certainty as 
to its absolute value. Especially the measurements in the blue part 
of the spectrum have here the greatest convincing power. 
Finally I wish to express my thanks to Miss C. M. PrrrrBoom 
assistant at the Physical Laboratory, Amsterdam, for her assistance 
in part of the experiments and of the caleulations and to Mr. J. 
VAN DER ZWAAL, mechanician of the Laboratory for his assistance in 
making the apparatus. 
10. In one respect the experiments can teach still something on 
the question whether the length of the moving water-column with which 
the calculations have been carried out, has been fairly well chosen. 
According to these Proceedings (Vol. 18, 401, 1915) 1 took for this 
length the distance between corresponding points of intersection of 
the axes of the O-shaped supplying-tubes with the axis of the ap- 
paratus. With the apparatus described in $ 2 I have been able to 
prove that this was the right length. Through the window JV stream- 
lines can be observed, if a vertical plane through the axis of the tube 
is illuminated. With an accuracy ‘of some millimeters we can indicate 
in which point the stream-lines become rather suddenly parallel to 
the axis of the tube “YD, while on the left of that point the fluid 
is nearly at rest. There cannot exist any doubt whether the motion 
must be reckoned from the point W indicated in the Figure. On the 
whole length of Fizmau’s tube i.e. 302 em. the inaccuracy of some 
millimeters in the determination of the place of |” is of no consequence. 
Physics. — “The specifie heat at low temperatures. U1. Measurements 
of the specific heat of solid nitrogen between 14° K. and the 
triple point and of liquid nitrogen between the triple point 
and the boiling point” By W. H. Kersom and H. KaMeRLINGH 
Onnes. Communication N°. 149a from the Physical Laboratory 
at Leiden. (Communicated by Prof. H. KAMERLINGH ONNBs). 
(Communicated in the meeting of January 29, 1916). 
§ 1. The investigation of the specific heat of condensed monatomic 
and di-atomic gases appears of special interest with a view to the 
conclusions, which may be drawn about the crystal structure from 
the comparison of the specific heat in the solid state for these two 
groups of substances. In particular the question arises whether for 
the last mentioned group of substances their di-atomie nature- does 
or does not show itself in the solid state also. 
§ 2. We began with the investigation of the specifie heat of 
