Page:Popular Science Monthly Volume 74.djvu/230

226 was poorly exposed. There is usually more or less difference in the resistance of individuals, but never as much as that apparently indicated, in the 72-hour culture. This and similar experiments confirm the results of Koernicke and others that the effect varies directly with the duration of exposure.

The relative effect of preparations of different activities is illustrated by the following typical experiment. Three sets, a, b and c, of six dry seeds of the white lupine were exposed to rays from sealed glass tubes of radium bromide by laying the tubes in contact with the hilum edges of the seeds. Care was taken to have the radium salt distributed evenly along the bottom of the horizontally placed tube. The activities of the preparations were: a, 1,800,000; b, 1,500,000; c, 10,0-00. A fourth set, d, served as a control. All exposures were for 91.5 hours. The seeds were then sown in soil in pots, and the comparative amounts of growth in the four cultures are shown in Fig. 4. The activity decreased from left to right. It is clearly demonstrated that the stronger the activity the greater the amount of retardation, under the conditions of the experiment.

An experiment to test the effect of a radioactive atmosphere on germination and growth was facilitated by the preparation by Mr. Lieber of a tube lined with the radium coating devised by him. This tube (T, Fig. 5) was connected with the upper tubulure of a glass bell-jar, resting air tight on a ground-glass plate. The lower tubulure was connected with an exhaust, so that air, entering the radium lined cylinder, carried with it into the bell-jar the radioactive