180 Professor Tyndall [April 8 



nearer approacli to the source of rays. Tested with this new reflector 

 the stronger vapours rose remarkably in sounding power. 



Improved manipulation was, I considered, sure to extract sounds 

 from rays of much more moderate intensity than those of the lime- 

 light. For this light, therefore, a common candle flame was substi- 

 tuted. Received and thrown back by the mirror, the radiant heat of 

 the candle produced audible tones in all the stronger vapours. 



Abandoning the mirror and bringing the candle close to the 

 rotating disk, its direct rays produced audible sounds. 



A red-hot coal, taken from the fire and held close to the rotating 

 disk, produced forcible sounds in a flask at the other side. 



A red-hot poker, placed in the position previously occupied by 

 the coal, produced strong sounds. 



The temperature of the iron was then lowered till its heat just 

 ceased to be visible. The intermittent invisible rays produced audible 

 sounds. 



The temperature was gradually lowered, being accompanied by a 

 gradual and continuous diminution of the sound. When it ceased to 

 be audible the temperature of the poker was found to be below that of 

 boiling water. 



As might be expected from the foregoing experiments anTincan- 

 descent platinum spiral, with or without the mirror, produced musical 

 sounds. When the battery power was reduced from ten cells to^three 

 the sounds, though enfeebled, were distinct. 



My neglect of aqueous vapour had led me for a time 'astray in 

 1859, but before publishing my results I had discovered the error. 

 On the present occasion this omnipresent substance had also to be 

 reckoned with. Fourteen flasks of various sizes, with their bottoms 

 covered with a little sulphuric acid, were closed with ordinary corks 

 and permitted to remain in the laboratory from December 23 to 

 January 4. Tested on the latter day with the intermittent beam, 

 half of them emitted feeble sounds, but half were silent. The 

 sounds were undoubtedly due, not to dry air, but to traces of aqueous 

 vapour. 



An ordinary bottle containing sulphuric acid for laboratory 

 purposes, being connected with the ear and placed in the intermittent 

 beam, emitted a faint, but distinct, musical sound. This bottle had 

 been opened two or three times during the day, its dryness being 

 thus vitiated by the mixture of a small quantity of common air. A 

 second similar bottle, in which sulphuric acid had stood undisturbed 

 for some days, was placed in the beam : the dry air above the liquid 

 proved absolutely silent. 



On the evening of January 7, Professor Dewar handed me four 

 flasks treated in the following manner : — Into one was poured a small 

 quantity of strong sulphuric acid ; into another a small quantity of 

 Nordhausen sulphuric acid ; in a third were placed some fragments 

 of fused chloride of calcium ; while the fourth contained a small 

 quantity of phosphoric anhydride. They were closed with well- 



