Vol.. 7, 1921 
PHYSICS: MILLIKA N AND BARBER 
13 
at 17 years. The insensible perspiration for these girls per kilogram of 
body weight per hour was as follows: 0.72 gram at 13 years, 0.71 gram at 
14 years, and 0.77 gram at 15 years. The respiratory quotients of groups 
of 12 girls each, about 7 to 8 hours after a light meal, were 0.81, 0.81, 
0.78, and 0.79. 
The basal heat production per kilogram of body weight per 24 hours 
decreases regularly with increasing age from 29.9 calories at 12 years 
2 months to 21.7 calories at 17 years. The heat production per square 
meter of body surface per 24 hours likewise decreases, but not so regularly, 
with increasing age, ranging from 928 calories at 14 years to 745 calories 
at 16 years. 
The metabolism of groups of young girls can be predicted from the 
general curve indicating the heat production per kilogram of body weight 
referred to age to within an average error of =±=3.1%. The prediction of 
the heat production per unit of body weight is somewhat better than that 
per unit of surface area. The curves representing the heat production 
per kilogram of body weight referred to weight and per square meter of 
body surface referred to weight for these groups of girls from 12 to 17 
years of age blend with remarkable uniformity with similar curves based 
upon the measurement of a large number of normal girls from birth to 12 
years of age. 
No influence of puberty or the prepubescent stage is clearly proven in 
any of the results. 
The details of the entire research are shortly to appear in the Boston 
Medical and Surgical Journal. 
ON THE REFLECTION AND RE-EMISSION OF ELECTRONS 
FROM METAL SURFACES: AND A METHOD OF MEASUR- 
ING THE IONIZING POTENTIAL OF SUCH SURFACES 
By R. A. Millikan and I. G. Barber 
Rybrson Physical Laboratory, University of Chicago 
Read before the Academy, November 16, 1920 
In making photo-electric measurements upon suspended droplets of 
mercury, one of us has repeatedly found that photo-electrons are always 
released when the droplet is illuminated by the ultraviolet line of wave- 
length 2536 Angstroms. On the other hand, it has been frequently 
proved that the molecules of mercury vapor are not ionized at all by these 
wave-lengths. The quantum of energy, then, which must be incident 
upon mercury in the liquid form in order to detach electrons from it is 
quite different from that required to detach electrons from the free atoms. 
It is to be expected from this fact that the "ionizing potential" also of 
liquid mercury will be quite different from that of the same element in 
