CHLOROPHYLL AND ENERGY 159 



points. It is easy to see, with the Geiger counter, how the 

 radioactive carbon is distributed. The property of radioactive 

 substances of producing an image on a photographic plate 

 can be used to obtain a radio-autograph from the sheet of 

 filter-paper. There the size and distribution of the spots 

 indicate the substances which contain radioactive carbon and 

 the proportion contained. 



These new techniques have enabled new theories to be 

 formulated from concrete data. 



