514 Mr. "W. H. Preece. On the Conversion of [Mar. 10, 



21. The plate e was replaced by white note paper. The sounds 

 were very faint but perceptible. 



22. It was replaced by thin cardboard, and the effect was nil. 



23. All these effects were produced equally well, whether the cases 

 were placed at 0, fig. 2, 400 centims. from the source of light, or 

 16 centims. from the rotating disk R, but in the latter case their 

 intensity was of course always increased. 



Hence it is abundantly evident that these sonorous vibrations are 

 due to the motions of the contained air, and not to those of the disk ; 

 that they are actually improved by the removal of the disk, that their 

 production is materially assisted by lining the surface of the containing 

 space with an absorbent substance, that they are dependent on the 

 heat rays that pass through, and that they disappear when the rays 

 are cut off from the air cavity by an athermanous diaphragm. 



24. Dr. Tyndall having shown in the paper already referred to, 

 that water vapour responded actively to these intermittent actions, a 

 clean empty one-ounce glass flask was taken and exposed to the inter- 

 mittent beams. No sound was produced. 



25. It was then filled with water vapour by pouring a small quantity 

 of water into it, and warming it in a flame, fair sounds of an intensity 

 25 were the result. 



26. The flask was filled with the dense smoke from burning 

 camphor, and the sounds were intensified considerably. 



27. The case (fig. 6) was taken and a glass plate 1*5 millims. thick 

 fixed in front of it as before. 



a. When the glass was dry, sounds were 20. 



b. When the glass was wetted on the inside, sounds were 25. 



28. Another clear one-ounce glass flask was taken. 



a. When clear, sounds = 0. 



b. When filled with tobacco smoke, sounds == 5. 



c. When filled with heavy camphor smoke, sounds = 30. 



29. One side of the flask was blackened on the outside, the other 

 side remaining clear. 



a. On exposing the clear side to the light fair sounds 25 were 

 obtained. 



b. On exposing the blackened side, no sounds were produced. 



30. The flask was blackened in the interior on one side only. 



a. When the blackened side was near the source, sounds = 25. 



b. When it was away from the source, sounds = 30. 



c. When the flask was cleaned, all sounds disappeared. 



31. A thin glass plate was now blackened on one side and placed in 

 front of the case, fig. 6. 



a. When the black surface was outside, no sounds were ob- 

 tained. 



