Science Record

42

Table 1 shows the times of survival in days after infection with tubercle It is seen that the tubercle bacilli screened. with 2.1, 7.4 and 95 cm

bacilli.

25 em

Fig.

Zcm

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S)

b

2 MX SS

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N@ B

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BNEN a

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1 Lead-box for exposing the

<

tubercle bacilli to cosmic radiation under

various thickness

of lead screen and for

screening it from radioactive rays.

Fig. 2

of lead showed a great decrease in virulence as only 2, 7 and 1 mice died 43, 3469 and 41 days after infection respectively. The remaining 45 mice are still living 80 days after infection. The analysis of variance of the other groups of mice (Table 2) shows that the difference between the mean times

~ of survival of mice injected with tuber-

cle bacilli screened with 3.2, 4.2, 5.3, 6.3 and 10.5 cm of lead and that of the control mice is highly significant. There is no significant difference between the means of the three control groups, i.e. the control mice were random samples from the same population.

In Fig. 2 the mean times of survival of the various groups of mice are plotted against the thickness of lead screen. It is seen that there is a sharp maximum at approximately 2 cm of lead corresponding to a maximum number of showers and a second broad maximum between 7 and 10.5 cm of

lead.

2 ——————= ¢m. Pb.

Mean times of survival of mice injected with Q,4 mg of tubercle bacilli exposed to cosmic radiation plotted against the thickness of lead screen..T, mean time of survival. in days.