﻿with Carbon Dioxide in the Solid State. 85 



property. And the effect in general may be augmented by the 

 friction of the current of gas against the surrounding air of low 

 temperature and saturated therefore with moisture. 



Production of Sound. — The experiment just described, 

 demonstrating that a piece of strongly compressed carbon 

 dioxide on mere contact with a copper plate may acquire a 

 sufficient electric charge to attract an electric pendulum, and 

 that this should be originated by little impacts from the metal, 

 seems to obtain fresh evidence from another curious fact I 

 often observed on putting this substance in contact with metals, 

 and which consists in a loud sound produced for some time. 

 On further investigation it was soon ascertained that this is 

 due to the thermal conductivity of the metal, acting by increas- 

 ing the rapid evaporation of the solid matter in different points 

 where the contact was best, and the gas evolved, more or less 

 prevented from easily escaping between the surfaces, was 

 alternately compressed and expanded, producing a vibratory 

 condition of the current of gas. A convenient way to perforin 

 this experiment is to take a disk or cylinder of carbon dioxide, 

 very highly compressed, and to lay down on it a small brass 

 sphere with a diameter of 1 cm. ; if the whole system is placed 

 on the resonance box of a tuning-fork (for instance that of Ut 4 

 answers very well), a loud sound of very high pitch makes 

 itself at once audible throughout the whole room ; if the 

 metal is pressed against the solid substance, it may even 

 become so loud as to be painful to the ear. It slowly dies 

 out as the metal grows too cold to accelerate the evaporation 

 in an effective way, but the sound is again restored when 

 the metal is heated for a moment. Things may be reversed : 

 that means, a cylinder or, still better, a lens of plano-convex 

 form may be set down on a metal surface, and if this consists 

 of a thin copper disk of 8 cm. diameter fixed in its centre, this 

 emits a loud sound, resembling the ringing of a bell. It is easily 

 perceived that the tone is produced at the place of contact wheie 

 the metallic sphere touches the extremely cold carbon dioxide ; 

 a little cavity is formed (as the metal is comparatively very hot), 

 and this is somewhat closed up by the sphere sinking slowly 

 down ; the occluded gas gets more tension till it can lift the 

 metal, when for a very brief moment it again sinks down, 

 and so on in rapid succession. Of course the same state of 

 things nearly occurs as with a steam-jet escaping downwards 

 from a conical aperture, closed up more or less by a little 

 sphere. It is well known, when high pressure is used, it is 

 not blown away, but remains in front of the opening in a very 

 rapid oscillating condition, and there is also a dull sound often 

 audible. 



