1856] WRITINGS OP JOSEPH HENRY. 415 



near the end of the fork, this cavity having previously been 

 adjusted to unison with the vibrations of the fork, gave an 

 audible sound when none could be heard by the unaided 

 ear. 



The tuning-fork was next placed upon a cube of India- 

 rubber, and this upon the marble slab. The sound emitted 

 by this arrangement was scarcely greater than in the case of 

 the tuning-fork suspended from the cambric thread, and 

 from the analogy of the previous experiments, we might at 

 first thought suppose the time of duration would be great, 

 but this was not the case. The vibrations continued only 

 about forty seconds. The question may here be asked. What 

 became of the impulses lost by the tuning-fork ? They were 

 neither transmitted through the India-rubber nor given off 

 to the air in the form of sound, but were probably expended 

 in producing a change in the matter of the India-rubber, or 

 were converted into heat, or both. Though the inquiry did 

 not fall strictly within the line of this series of investigations, 

 yet it was of so interesting a character in a physical point of 

 view to determine whether heat was actually produced that 

 the following experiment was made. 



A cylindrical piece of India-rubber about an inch and a 

 quarter in diameter was placed in a tubulated bottle with 

 two openings, one near the bottom and the other at the top. 

 A stuffing-box was attached to the upper opening, through 

 which a metallic stem with a circular foot to press upon the 

 India-rubber was made to pass air-tight. The lower opening 

 was closed with a cork, in a perforation of which a fine glass 

 tube was cemented. A small quantity of red ink was placed 

 in the tube to serve as an index. The whole arrangement 

 thus formed a kind of air-thermometer, which would indicate 

 a certain amount of change of temperature in the enclosed 

 air. On the top of the stem the tuning-fork was screwed, 

 and consequently its vibrations were transmitted to the rub- 

 ber within the bottle. The glass was surrounded with several 

 coatings of flannel to prevent the influence of external tem- 

 perature. The tuning-fork was then sounded, and the vibra- 

 tions were kept up for some time. No reliable indications 



