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PROCEEDINGS OP THE AMERICAN ACADEMY 



the oven, presented before the opening in the thermograph, and the 

 deflection noted. The specimens were then returned to the oven, and 

 some time allowed to elapse before the experiment was repeated. In 

 this way as many measures were obtained from each specimen as 

 could be made in the course of a day, i. e. from ten to thirty. The 

 method seems far from perfect, but it was the best of several that 

 were tried. 



To reduce the results to the lamp-black standard, a very careful 

 comparison was made between the radiating powers of blackened and 

 unblackened faces of the quartz. The temperature at which the 

 measures were made was near 100°. The results are given in the 

 following table, the radiation from a blackened surface of quartz being 

 taken as 100. 



An inspection of the table shows a remarkable uniformity in radiat- 

 ing power for materials so diverse in character. The large number of 

 volcanic rocks was selected, because it seems probable, from its physi- 



