Award of the Boijle Medal to Professor John Jobj. 143 



possible to reproduce with accuracy tlie colours of nature on a transparent 

 plate. His work on the influence of temperature on the sensitiveness of the 

 photographic plate and his theory that the latent image arises from plioto- 

 electric ionization may also be mentioned among his contributions to the 

 science of photography. 



In 1898 he showed how the sodium content of tlie ocean could be used as 

 a measure of geological time, and so was able to calculate the period whicli 

 has elapsed since the beginning of denudation of the earth's surface, and to 

 establish a chronological unit of importance in cosmology. He also introduced 

 the sodium content of tlie ocean as a factor enabling calculation to be extended 

 to many far-reaching but hitherto indeterminate problems of solvent denuda- 

 tion : tor instance, the mass of the parent rock, the mass of the derived 

 sediments, and of the sub-oceanic deposits. 



The theory of sedimentation has also been advanced by his researches on 

 the cause and effects of electrolytic precipitation. 



By many researches he has laid tlie sciences of Petrology and Mineralogy 

 under obligations to him. We would specially notice his invention of a 

 polarizer whereby the value of birefringence as a means of identification is 

 increased, and his application of the microscope to the determination of the 

 quality of paving-sets and road-metal. 



In connexion with the new subject of radioactivity, the same happy 

 combination of a knowledge of physics and geology, which he had already 

 used with such effect in earlier researches, has enabled him not only to 

 advance our knowledge of the properties of radioactive substances, but also to 

 apply this knowledge to the explanation of geological phenomena of the most 

 far-reaching importance. These investigations are summarized in his work 

 on " Radioactivity and Geology." His explanation of pleochroic haloes in 

 rocks as due to radioactivity removed a long outstanding difficulty, and leads to 

 important conclusions as to the non-existence of alpha-radiation from common 

 elements, and as to the absence or scarcity of radioactive substances other 

 than those already known, and also affords a means of identifying quantities 

 of radioactive matter in situ far less tlian can be detected by other methods. 



By the determination of the tliorium content of over 150 igneous and 

 sedimentary rocks (using a method of his own), he has for the first time 

 established a probable mean value for the distribution of that element in the 

 various surface-materials of the earth. 



To the literature of speculative philosophy Joly has also contributed. Here 

 may be mentioned, more especially, an essay on the " Prematerial Condition 

 of the Universe," and one (" The Abundance of Life ") on the physical laws 

 underlying organic evolution. 



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