. Adventures in radioisotope research;. Radioactive tracers; Radiobiology. THE VELOC'ITV OF DISSOLUTION OF MOLECULAK LAYERS 95 of the same lead chloricU^ 44 per cent had already dissolved in 1 hr, representing 44 per cent of the saturation concentration since the liquid volume amounted to 25 cm^ and 250 mgni are soluble in tliis volume. It is seen, therefore, that tlie velocity of exchange belween the 1 wo phases is small compared Avitli the velocity of solubility. Il will be seen later that in the case of a molec^ular layer the exchange velocity becomes much larger when expressed on a percent
. Adventures in radioisotope research;. Radioactive tracers; Radiobiology. THE VELOC'ITV OF DISSOLUTION OF MOLECULAK LAYERS 95 of the same lead chloricU^ 44 per cent had already dissolved in 1 hr, representing 44 per cent of the saturation concentration since the liquid volume amounted to 25 cm^ and 250 mgni are soluble in tliis volume. It is seen, therefore, that tlie velocity of exchange belween the 1 wo phases is small compared Avitli the velocity of solubility. Il will be seen later that in the case of a molec^ular layer the exchange velocity becomes much larger when expressed on a percentage basis but is still smaller than the velocity of solubility, and thus a diminution of the velocity of dissolution of a substance can still be detected by the presence of its isotope in the solution. To this end we have compared the amounts of ThB collected on quartz which dissolved in water and in a saturated PbClg solution, under the same conditions, in 1 hr. There w^as only a small difference since in the first case 79 per cent dissolved while in the second the value was 75 per cent. Shorter experimental times were then chosen and thus the presence of lead ions in the solution had a very considerable effect on the velocity of solubility of ThB. These experiments were performed with the same apparatus used for obtaining the values recorded in Table 1. TaULI-; 4. — TIME 60 SGC Bismuth isotope ThC Lead isotope ThB .Vmount ilissolved in 10 - 3 N HXO., (%) Amount dissolved in 10—' N HXOj s:iturated with Pb(X03)2 (%). fi4 67 (?-'; The presence of lead ions in the solution diminishes the velocity of solubility of the lead isotope but not that of the bismuth isotope. It is a fortunate circumstance that ThB and ThC are simultaneously present in the same place on the quartz surface, and when, in spite of this, the velocity of solubility of only one is affected, this means that there is a specific effect due to the addition of the appropriate element ; for example
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