ISOTOPES — ATEN AND HEYN 219 



phorus added is the same everywhere. If phosphorus disappears 

 from the melt into the sUig floating on the surface or into the lining 

 of the crucible this takes place to an equal degree with the natural 

 and with the radioactive element. The decrease in the percentage of 

 phosphorus in the melt can thus be determined merely by ascertaining 

 the decrease in the radioactive phosphorus. This is extremely simple, 

 since it is only necessary to measure the radioactivit}; of a sample of 

 the melt, which can be done very easily with an electrometer or an 

 electron counter.^ 



Second example. — In many factories the workers come into contact 

 with mercury, and it is known how harmful the regular inhalation of 

 mercury vapor can be ; in course of time a concentration of more than 

 10"* gram of mercury per m^ of air already becomes injurious to health. 

 It is a difficult problem to detect the presence of mercury in such 

 minute proportions, because chemical analyses are unavailing in such 

 cases. In a certain case which occurred in the manufacture of tubular 

 luminescent lamps in an American factor}?, where the lamps were 

 filled with mercury vapor at a low pressure, a glass side-tube con- 

 taining a small drop of mercury had to be "blown" onto the lamp, and 

 inevitably the glass blower inhaled a very small quantity of mercury 

 vapor into his lungs. In order to determine how much was inlialed 

 a number of tests were carried out with a volume of 2 liters of air that 

 had been in contact with the drops of mercury under exactly the same 

 conditions as in the manufacturing process, this being drawn off by 

 suction and passed over a metal plate kept at the temperature of liquid 

 air. Practically all the mercury in the air condensed on the plate. 



The mercury used for the experiments contained a known, small 

 percentage of a radioactive mercury isotope. The radioactivity of the 

 plate, which was quite simple to measure after the experiment, gave 

 an indication of the amount of mercury contained in the air which 

 had passed over the plate. In this way a concentration of 5 X 10"^ g/m^ 

 could be detected. Average mercury concentrations were found of 

 about 10"^ and in one case about 4 X 10-^ g/m^, from which it was con- 

 cluded that in the manufacturing process in question the glass blower 

 ran no danger of poisoning.^ 



Third example. — When a piece of metal is fused with a radioactive 

 lead isotope in an atmosphere of hydrogen and then allowed to crystal- 

 lize again, there are two possibilities. In some metals, such as thal- 

 lium and magnesium, lead is soluble to a considerable percentage, the 

 lead atoms being uniformly distributed in the grains and the poly- 

 pi An electron counter was described, e. g., by A. Bouwers and F. A. Heyn, in Philips 

 Techn. Rev., vol. 6, p. 75, 1941. In the measurements allowance must of course be made 

 for the natural decline in radioactivity with time. We shall return to such practical 

 details on a later occasion. 



» J. W. Irvine and C. Goodman, Journ. Appl. Phys., vol. 14, p. 496, 1943. 



