COMMEMOBATIVE ADDRESS 7 



Joule, not only in the magnitude of the quantities to be observed, but 

 especially in the greater attention given to the matter of thermometry. 

 In common with Joule and other previous investigators, he made use 

 of mercury thermometers, but this was only for convenience, and they 

 were constantly compared with an air thermometer, the results being 

 finally reduced to the absolute scale. By experimenting with water at 

 different initial temperatures he obtained slightly different values for 

 the mechanical equivalent of heat, thus establishing beyond question 

 the variability of the specific heat of water. Indeed, so carefully and 

 accurately was the experiment worked out that he was able to draw 

 the variation curve and to show the existence of a minimum value at 

 30 degrees C. 



This elaborate and painstaking research, which is now classical, was 

 everywhere awarded high praise. It was published in full by the Amer- 

 ican Academy of Arts and Sciences with the aid of a fund originally 

 established by Count Eumford, and in 1881 it was crowned as a prize 

 essay by the Venetian Institute. Its conclusions have stood the test 

 of twenty years of comparison and criticism. 



In the meantime, Rowland's interest had been drawn, largely per- 

 haps through his association with his then colleague, Professor Hast- 

 ings, toward the study of light. He was an early and able exponent 

 of Maxwell's Magnetic Theory and he published important theoretical 

 discussions of electro-magnetic action. Recognizing the paramount im- 

 portance of the spectrum as a key to the solution of problems in ether 

 physics, he set about improving the methods by which it was produced 

 and studied, and was thus led into what will probably always be re- 

 garded as his highest scientific achievement. 



At that time, the almost universally prevailing method of studying 

 the spectrum was by means of a prism or a train of prisms. But the 

 prismatic spectrum is abnormal, depending for its character largely 

 upon the material made use of. The normal spectrum as produced by 

 a grating of fine wires or a close ruling of fine lines on a plane reflect- 

 ing or transparent surface had been known for nearly a hundred years, 

 and the colors produced by scratches on polished surfaces were noted 

 by Eobert Boyle, more than two hundred years ago. Thomas Young 

 had correctly explained the phenomenon according to the undulatory 

 theory of light, and gratings of fine wire and, later, of rulings on glass 

 were used by Fraunhofer who made the first great study of the dark 

 lines of the solar spectrum. Imperfect as these gratings were, Fraun- 

 hofer succeeded in making with them some remarkably good measures 



