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 w^ere 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 pow'er 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 



> Thanks are diie to Mr. Hugo Lieber and Dr. Theodore Lyman for the loan of 

 radium. 



' Rutherford, E., Radioactivity. University Press, Cambridge, 1905, xi + 580 pp. 



