594 
and only the slow automatic extension and flexion movement with 
a rhythm of 2°/, per second remained visible, often during 30 
seconds and longer. 
All the records shown in this paper are obtained from the patellar 
tendon-reflex. Yet their significance is more general, as also other 
exaggerated deep reflexes sometimes show changes of a similar 
nature. 
Physics. — “The specifie heat at low temperatures. 1. Measurements 
on the specific heat of lead between 14° and 80° K. and of 
copper between 15° and 22° K.” By W. H. Kersom and H. 
KAMERLINGH ONNEs. Communication N°. 148 from the Physical 
Laboratory at Leiden. (Communicated by Prof. H. KAMERLINGH 
ONNEs). 
(Communicated in the meeting of October 31, 1914). 
§ 1. Introduction. Soon after the methods of obtaining baths 
of temperatures which are accurately known and can be kept con- 
stant for a long time, in the range between the boiling point and 
the melting point of hydrogen, had been worked out *) in the eryo- 
genic laboratory at Leiden, a series of investigations on the calori- 
metry at very low temperatures was started there. Dewar’s measure- 
ments on the mean specific heat of different substances between 
the temperature of liquid air and the boiling-point of hydrogen ’) 
had particularly drawn attention to the interest of those investigations. 
The continuation of his experiments in the still lower region of 
temperatures mentioned above seemed very desirable *). As was done 
by Dewar the series of investigations in this direction was begun 
by determinations of the heat of vaporisation of hydrogen. A report 
on the first results of those determinations was given at the Dutch 
1) Cf. specially H. Kamertinen Onnes, Leiden Comm. NO. 947 (Proceedings 
Sept. '06) and H. Kamertinen Onnes, C. BRAAK and J. Cray, Leiden Comm. NP, 101a 
(Proceedings Dec. '07). 
2) J. Dewar, Proc. Roy. Inst. March 25, 1904, Proc. Roy. Soc. A 76 (1905) 
p. 325, later more extensively between the boiling point of nitrogen and that of 
hydrogen: Proc. Roy. Soc. A. 89. (1913) p. 158. 
35) This was pointed out at the Ist International Congress of Refrigeration at 
Paris 1908. H. Kameruincu Onnes, La liquéfaction etc. Note J. Sur les expériences 
a faire aux températures très basses. Leiden Gomm. Suppl. N°, 21a p. 29, 
