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was believed to be M).2 microampere, but the effective bombarding current may have been severalfold greater because of multiple traversals of the target. On the basis of comparative yields, the cross section appears to be ^lO" 4 barn.Sheline and Johnson have mentioned the potential significance of Mg 28 as a tracer, particularly in photosynthesis and general biochemistry. It should also be of value in the investigation of metallurgical systems, of geochemical problems, and of organomagnesium compounds such as Grignard reagents. While the above yield is not great, it is ample for producing a tracer for experiments of several days duration. We plan to seek target materials and conditions that will give higher yields and to develop chemical procedures for isolating Mg 28 in carrier-free condition. D ETAILS of the lateral structure of individual air showers have been observed 1 by using a 20-channel electron-pulse ionization chamber which took 20 density samples from each shower. The geometry is shown in plan in Fig. 1, where each area represents the region from which an individual collecting wire gathered electrons. The pulses were separately amplified, displayed on 20 cathode-ray tubes, and photographed. The last ' stage of amplification was made nonlinear so that a large range of densities could be accurately measured. Amplifier calibrations were made by pulsing the shell of the chamber and stability of gain proved satisfactory. The absolute value of ionization was determined in the customary manner from Po a-particle ionization. Precautions such as purification of the argon, experimental and theoretical evaluation of the pick-up between detection areas, etc., were taken. The recording system was actuated by a fourfold coincidence in a simple Geiger-counter array. During 109 hours of operation at an altitude of 280 meters, events were recorded in which the density distributions over the array of wires in the chamber were of the following types: (1) flat with only small fluctuations from the average (consistent with those expected from the statistics of independent events) (events 3 and 4); (2) increasing or decreasing functions (event 9); (3) markedly peaked (events 5 and 8); and (4) flat with rather marked and systematic fluctuations from the average (event 13) (see Figs. 2 and 3). These events have been interpreted, respectively, to be the result of showers whose axes hit (1) relatively far away from the chamber; (2) near the end of the chamber with small 4 7 CM CM N; 15 lo\ 5\ N; 15 lo S loo SO\ Hi 15 ,/V/ _ ., K. x -T?/V£. V X * to 50 JOO 150 CM o SHOWER AXIS FIG. 1. Plan of the detection areas. FIG. 2. Individual showers; number of electrons Ni within the detection areas versus position of the areas. In 3 and 4 the dotted lines represent (Ni)$ and the dashed lines cr, the rms deviation from the average. The curve in 9 represents a shower of local size II =3.5X10* electrons with 5=28.5 cm. The curve in 5 represents II =5.6 X10 3 with 8 =23.5 cm.values of 5; (3) inside the chamber collecting area; and (4) insid...

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