220 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



crystalline metal obtained. In other metals, on the other hand, such 

 as bismuth, tin, antimony, silver, gold, copper, and nickel, in which 

 lead is practically insoluble, the radioactive lead is situated on the 

 boundaries of the grains. When a microscopic preparation of the 

 metal is made and laid on a photographic plate for several hours after 

 development, the plate will be found to be blackened at those places 

 where it lay against radioactive particles, thus where lead has been 

 deposited. By means of such an "autoradiogram," of which plate 1, 

 figure 1 is an illustration, it is possible in the first place to ascertain 

 whether and to what degree the added lead is soluble in the metal ; if 

 the lead is entirely dissolved the entire surface of the photographic 

 plate is uniformly blackened; if the lead does not dissolve, or only 

 partially, the radiogram shows up very nicely also the boundaries of 

 the grains (pi. 1, fig. 1). Thus the isotopic method can also in this 

 case furnish valuable information about the changes taking place in 

 the structure of the metal upon recrystallization and in rolling.* 



Fourth exaTfiple. — Friction between two metal surfaces is due partly 

 to adhesion, the result being that when the surfaces slide over each 

 other extremely small particles of metal are torn out of one and taken 

 up in the other. This exchange of metal may take place to such an 

 extent that two surfaces become, as it were, welded together (the 

 familiar seizing). The quantity of material thus transferred from 

 one metal to the other is a measure of the contribution of this effect 

 to the total force of friction. In general it is a question of very small 

 amounts which chemically can hardly be detected at all. An investi- 

 gation has now been carried out with the help of a radioactive tracer." 

 One metal surface was "activated," i. e., it consisted for a small 

 part of atoms of a radioactive isotope of the metal. After this sur- 

 face had been made to slide over the second, nonactivated metal sur- 

 face, the latter also showed a certain amount of radioactivity. 

 Amounts of 10-^° gram of transferred metal were detected in this 

 way, and, what is more, by means of a radiogram, as described in the 

 preceding example, also the distribution of the transferred material 

 on the surface could be studied. From the radiogram shown in plate 1, 

 figure 2, (Z, it may be concluded, for example, that in this experiment 

 the sliding of the two metal surfaces over each other was not con- 

 tinuous but took place in small jerks. By this method the influence of 

 all kinds of factors, such as the pressure, the hardness of the surface, 

 etc., on the transfer of material can be studied, as also the effect of a 

 lubricant. (See pi. 1, fig. 2, h.) 



«G. Tammann and G. Bandel, Zeitschr. Metallk., vol. 25, pp. 153 and 207, 1933. 

 ■ B. W. Sakmann, J. T. Burwell, and J. W. Irvine, Journ. Appl. Phys., vol. 15, p. 495, 

 1944; J. N. Gregory, Nature (London), vol. 157, p. 444, Apr. 6, 1946. 



