378 On the Change from Radiant Heat and Light into Sound. 



gases and vapours enclosed in sealed tubes, he was able to 

 produce musical tones from the substances*. It was thus 

 proved that vaporous bodies as well as solids were able to 

 convert thermal undulations into sonorous ones. Other expe- 

 rimenters have extended these observations")". The important 

 result of these experiments is that the molecules of certain 

 bodies possess the power of transforming the undulations of 

 short period which fall upon them into those of long period. 

 Expressed briefly, some bodies are capable of transforming 

 inaudible undulations into audible ones. Now refractive 

 bodies have the power of transforming non-luminous calorific 

 rays into luminous ones; and this passage from the one state 

 into the other is called calorescence. Again, when the ultra- 

 violet rays are permitted to fall upon certain substances they 

 render the substance luminous. In other words, the substance 

 has transformed the non-luminous undulations into luminous 

 ones. To this change the term fluorescence has been applied. 

 Thus we have: — 



Fluorescence — change from more rapid into less rapid un- 

 dulations. 



Calorescence — change from less rapid into more rapid un- 

 dulations. 



In the new phenomena we have again a change from more 

 rapid into less rapid, which is very similar to that which occurs 

 in fluorescence. To the change which takes place in this case, 

 therefore, I venture to propose the term " Sonorescence." A 

 body such as hard rubber, in which it was first noticed, would 

 be called a sonorescent body, just as sulphate of quinine is a 

 fluorescent body. The points of similarity between the three 

 phenomena are sufficiently evident ; the chief points of differ- 

 ence are the following : — 



(1) The change in sonorescence is greater in magnitude 

 and is different in kind from that in the other two cases. In 

 fluorescence and calorescence the particles oscillate in every 

 case across the direction of propagation, but in sound we 

 believe them to move in the direction of propagation. Thus, 

 in the new phenomena we have a change of direction and also 

 a change in frequency of the oscillations. 



(2) In fluorescence and calorescence the result is in both 

 cases to produce luminous undulations from non-luminous 

 ones. In sonorescence, on the contrary, we produce sonorous 



* " Action of an Intermittent Beam of Radiant Heat upon Gaseous 

 Matter," read before the Royal Society, Jan. 13, 1881. 



t More especially ought Prof. W. C. Rontgen to be mentioned. This 

 gentleman seems to have anticipated some of Tyndall's identical experi- 

 ments. See Phil. Mag. April 1881. 



