Smithsonian Report. 1947. — Aten and Heyn 



Plate 1 



I. Radiogram of tin witii a radiouftive lead isotope (thorium B) dej)osited at the 

 grain boundaries of the polvcrvstalline metal. Magnified about .six times. 

 (From G. Tammann and G. Bahdel, Zeitschr. Metallk., vol. 25, p. 154, 1933.) 



J .§: .a ii fi * 



, >, 



Radiograms of a metal surface after it has been rubbed with a piece of metal 

 containing a radioactive component. The radioactivity of the siu'face shows 

 there had been a transfer of material. This explains to a large extent the 

 friction set up when one surface slides over another. The illustration relates 

 to the friction of lead on steel: a, with no lul)ricant; b, with lubricant. Magni- 

 hed about six times. (From J. N. Gregorv, Nature (London), vol. 157, p. 444, 

 1946.) 



