38 THE POPULAR SCIENCE MONTHLY. 



placed the flask in cold water until its temperature was reduced from 

 about 90 to 10 C, fully expecting that the sound would vanish at 

 this temperature ; but, notwithstanding the tenuity of the vapor, the 

 .sound extracted from it was not only distinct but loud. 



Three empty flasks, filled with ordinary air, were placed in a freez- 

 ing mixture for a quarter of an hour. On being rapidly transferred 

 to the intermittent beam, sounds much louder than those obtainable 

 from dry air were produced. Warming these flasks in the flame of a 

 spirit-lamp until all visible humidity had been removed, and afterward 

 urging dried air through them, on being placed in the intermittent 

 beam the sound in each case was found to have fallen almost to silence. 

 Sending, by means of a glass tube, a puff of breath from the lungs 

 into a dried flask, the power of emitting sound was immediately re- 

 stored. When, instead of breathing into a dry flask, the common air 

 of the laboratory was urged through it, the sounds became immediately 

 intensified. I was by no means prepared for the extraordinary deli- 

 cacy of this new method of testing the athermancy and diathermancy 

 of gases and vapors, and it can not be otherwise than satisfactory to 

 me to find that particular vapor, whose alleged deportment toward 

 radiant heat has been so strenuously denied, afiirming thus audibly its 

 true character. 



After what has been stated regarding aqueous vapor, we are pre- 

 pared for the fact that an exceedingly small percentage of any highly 

 athermanous gas diffused in air sufiices to exalt the sounds. An acci- 

 dental observation will illustrate this point. A flask was filled with 

 coal-gas, and held bottom upward in the intermittent beam. The 

 sounds produced were of a force corresponding to the known absorp- 

 tive energy of coal-gas. The flask was then placed upright, with its 

 mouth open upon a table, and permitted to remain there for nearly an 

 hour. On being restored to the beam, the sounds produced were far 

 louder than those which could be obtained from common air.* 



Transferring a small flask or a test-tube from a cold place to the 

 intermittent beam, it is sometimes found to be practically silent for a 

 moment, after which the sounds become distinctly audible. This I 

 take to be due to the vaporization by the calorific beam of the thin 

 film of moisture adherent to the glass. 



My previous experiments having satisfied me of the generality of 

 the rule that volatile liquids and their vapors absorb the same rays, I 

 thought it probable that the introduction of a thin layer of its liquid, 

 even in the case of a most energetic vapor, would detach the effective 

 rays, and thus quench the sounds. The experiment was made, and 

 the conclusion verified. A layer of water, formic ether, sulphuric 

 ether, or acetic ether, one eighth of an inch in thickness, rendered the 

 transmitted beam powerless to produce any musical sound. These 



* The method here described is, I doubt not, applicable to the detection of extremely 

 small quantities of fire-damp in mines. 



