CONGDON: EFFECTS OF RADIUM ON LIVING SUBSTANCE.— I. 349 
thousandth the strength of the pure bromide was the source of the 
radiations. It was contained in a cubical cell of lead (Fig. 1) the walls 
of which were 5 mm. thick. To eliminate so far as possible other 
influences than those of the radium, the experiments were conducted 
in a constant-temperature chamber (Fig. 3), and the eggs were kept 
moist by an irrigating device (compare Fig. 4). The eggs were 
supported above the cell on a sheet of filter paper parallel to the 
surface of the radium. The intensity of the beta radiations was con- 
trolled by varying the space between radium and eggs. By multi- 
plying the number expressing decrease of intensity due to the spread- 
ing of the radiations (based on radiographs at different distances 
above the radium) by the factor for absorption of beta radiations in 
the air,” quantities were obtained expressing the relative strength of 
treatment in the different experiments. The length of exposure was 
twenty-four hours. The decrease of penetrating power of the radia- 
tions at the different heights due to absorption by the air was 
negligible. 
The material for a single exposure consisted of from two hundred 
to four hundred fly eggs, which were removed to the centers of two 
small moist pieces of filter paper, an equal number on each piece, 
within two hours after being laid. The filter paper bearing one set was 
supported on a light wooden frame (carrier) at the desired distance 
from the radium (6, Fig. 4); the other, protected from the radiations 
by a mass of lead two inches thick, was kept in the same thermo- 
stat in which the exposure was going on. An automatic irrigating 
device (Fig. 4, d) prevented the eggs from drying. The larvae began 
to come out of the egg cases soon after removal and were usually 
- nearly all hatched in twenty-four hours. The average periods of 
incubation for the exposed and control eggs were never so different 
that the two sets did not well overlap in time of hatching. A com- 
parison of the number of eggs that hatched out from the exposed box 
during the period of the experiment with the number that hatched 
from the control during the same period, afforded a basis for an ap- 
proximate expression of the rate of growth of irradiated eggs. The 
comparison was made at the time when approximately half of the 
controls were hatched. It may be objected that time of hatching 
does not necessarily express amount of growth, and has to do only 
with the casting off of the egg coat itself. Such an explanation is 
1'Thanks are due to Mr. Hugo Lieber and Dr. Theodore Lyman for the loan of 
radium. 
* Rutherford, E., Radioactivity. University Press, Cambridge, 1905, xi + 580 pp. 
