v] RATE OF EMISSION OF ENERGY 161 
depended upon the age of the radium preparation pointed to the 
conclusion that the phenomenon of heat emission of radium was 
connected with the radio-activity of that element. It had long 
been known that radium compounds increased in activity for about 
a month after their preparation, when they reached a steady state. 
This increase of activity is due to the continuous production by the 
radium of the radio-active emanation or gas, which is occluded in 
the radium compound and adds its radiation to that of the radium 
proper. It thus seemed probable that the heating effect was in 
some way connected with the presence of the emanation. Some 
experiments upon this point have been made recently by Ruther- 
ford and Barnes. In order to measure the small amounts of heat 
emitted, a form of differential air calorimeter was employed. Two 
equal glass flasks of about 500 c.c. were filled with dry air at 
atmospheric pressure. These flasks were connected through a 
glass U-tube filled with xylene, which served as a manometer 
to determine any variation of pressure of the air in the flasks. 
A small glass tube, closed at the lower end, was introduced into 
the middle of each of the flasks. When a continuous source of 
heat was introduced into the glass tube, the air surrounding it was 
heated and the pressure was increased. The difference of pressure, — 
when a steady state was reached, was observed on the manometer 
by means of a microscope with a micrometer scale in the eye- 
piece. On placing the source of heat in the similar tube in the 
other flask, the difference in pressure was reversed. In order to 
keep the apparatus at a constant temperature, the two flasks were 
immersed in a water bath, which was kept well stirred. 
Observations were first made on the heat emission from 30 
milligrams of radium bromide. The difference in pressure observed 
on the manometer was standardized by placing a small coil of wire 
of known resistance in the place of the radium. The strength of 
the current through the wire was adjusted to give the same differ- 
ence of pressure on the manometer. In this way it was found that 
the heat emission per gram of radium bromide corresponded to 
65 gram-calories per hour. Taking the atomic weight of radium 
as 225, this is equivalent to a rate of emission of heat from one 
gram of metallic radium of 110 gram-calories per hour. 
1 Nature, Oct. 29, 1903. Phil. Mag. Feb. 1904. 
