138 Dr. E. Warburg on the Heating produced in 



conical by a quartz lens is projected upon a concentrated ethereal 

 solution, all the rays capable of developing fluorescence are ab- 

 sorbed at the surface, so that no cone of light is visible in the 

 solution ; but with a dilute solution a brilliant green cone is pro- 

 duced. The colours of the ethereal solution and its fluorescence 

 bear a remarkable resemblance to those of uranium-glass, but 

 with this difference, that when the fluorescent light is examined 

 in the spectroscope, while the fluorescent spectrum of uranium- 

 glass is, as is well known, discontinuous, that of fluoraniline is 

 continuous. 



As the investigation of this subject cannot be continued for 

 some time to come, it has been thought desirable to publish the 

 above imperfect note, that other experimenters may have the be- 

 nefit of the results hitherto obtained. 



Hadham House, Upper Clapton, 

 July 19, 1869. 



Postscript, July 21. — Since the above paper was written, the 

 author has discovered, in the aniline-red made from stannic 

 chloride, another fluorescent substance associated with fluorani- 

 line. The fluorescent spectrum consists of red, a very bright 

 green band, and some blue only. To the unassisted eye the fluo- 

 rescence has a cold blue tint. 



XVI. On the Heating produced in Solid Bodies when they are 

 Sounded. By Dr. E. Warburg*. 



IN the twenty-fourth volume of PoggendorfPs Annalen, Wil- 

 liam Weber mentions that his attention was excited by the 

 difference which bodies exhibit in the rapidity with which their 

 sound fades away. He shows that the resistance of the air, which 

 must diminish the amplitude the more rapidly the smaller the 

 mass of the body upon which it acts, is inadequate to explain 

 this phenomenon, and he arrives at the conclusion that it must 

 have its origin in the special nature of the substance. 



As a matter of fact, the sound of lead fades away more rapidly 

 than that of steel, while the density of lead is far greater than 

 that of steel. 



From these considerations, part of the vis viva of the vibrations 

 must be consumed in the interior of the sounding body ; and the 

 conclusion is obvious that it is here changed into heat. This 

 portion will be greater in the case of those bodies in which, as 

 in lead, the sound rapidly fades away — that is, only impart a 

 small amount of the motion to the surrounding medium. 



* Translated from the Berliner Monatsbericht for February 1869. 



