15 C. Solubility of Sr90 Deposited in Rains The above indirect evidence for the solubility of the Sr90 activity in fallout is confirmed by the analysis and masurement of the insoluble residues from Chicago rains, The insoluble residues from two active Chicago rains, CL 320 and CL hO7 & 1,08, initially separated by filtration, were fused with NagC03 and dissolved in HCl. Following a fuming nitric acid separation of Sr, the samples were reserved for yttrium-90 growth and milked for the ¥70 activity. The results are showm in Table 3 together with the Sr?0 activity of the water soluble fraction, Table 3 Solubility of Sr90 in Rains 320 CL 07-8 1, Total Sr70 Activity in Solution (DPM) 49.2 + 0.5 29.9 + 1.5 2. Total Sr90 Activity in Insoluble Residue (DPM) =0.6 0.7 + 0.1 3. Percent of Total Activity in Insoluble Residue £1,2h 2.3 + OL% De Chicago Fallout History from Rain and Soil Data . The Sr90 gata for Chicago rains and Chicago Milkshed soils are summarized in Figure l; which gives the approximate total soil burden of Sr70 and its change with time over the three year period, January 1953 to December 1955. The curve is normalized to the average available Sr?0 in six typical Chicago Milkshed soils collected on September 29 and 30, 1955, The total monthly fallout in Chicago rains are taken from Tables 1 and 2 above, and the method of measurement and data are discussed in the accompanying section, The Chicago Milkshed soil data are discussed below, The average available Sr9° observed for the sams six soils samples in late September 1953 is also plotted, The agreement in this case may be partly