March 24, 1881J 



NATURE 



497 



The lens was now removed from the front of the case, and it 

 was replaced by a movable glass plate (I '5 millims. thick) ; the 

 sounds were the same, but they gradually ceased on gradually 

 uncovering the front opening of the case, so as to give the air 

 room to expand. 



The glass plate was replaced by a heavy rigid plate of rock-salt 

 13 millims. thick, and the sounds were equally loud. The plate 

 was replaced by white note-paper. The sounds were very faint, 

 but perceptible. It was replaced by thin cardboard, and the 

 effect was nil. 



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 

 sm-face 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. 



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 intermittent beams. No sound was produced. 



It was then filled with water-vapour by pouring a small 

 quantity of water into it, and warming it in a flame ; fair sounds 

 were the result. 



The flask was filled with the dense smoke from burning 

 camphor, and the sounds were intensified considerably. 



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

 no sounds were heard. When filled with tobacco-smoke fair 

 sounds, but when filled with heavy camphor smoke very loud 

 sounds were obtained. One side of the flask was blackened on the 

 outside, the other side remaining clear. On exposing the clear 

 side to the light fair sounds were obtained, but on exposing the 

 blackened side, no sounds were produced. The flask was 

 blackened in the interior on one side only. When the blackened 

 side was near the source fair sounds, and when it was away from 

 the source better sounds were heard. When the flask was 

 cleaned all sounds disappeared. A thin glass plate was now 

 blackened on one side and placed in front of the case. When 

 the black surface was outside no sounds were obtained. When 

 the black surface was inside good sounds were the result. 

 When the glass was cleaned the sounds became still better. An 

 ebonite plate was similarly treated. When the blackened surface 

 was outside fair soands were obtained. When the blackened 

 surface ^^•as inside very poor sounds were the result. 



This being an anomalous result, several experiments were now 

 made to test the behaviour of opaque and transparent bodies, 

 when used as disks, for while in the previous experiments the 

 effect was greatest when the blackened surface faced the interior, 

 here we find the opposite result produced, viz., tlie greatest 

 effect was produced when the blackened surface was on the 

 exterior. 



Several experiments were then made, from which it appeared 

 that transparent bodies behave in an opposite way to opaque 

 bodies. Glass and mica can be rendered athermanous and silent 

 by making the carbon deposit sufiicientiy thick. Zinc, copper, 

 and ebonite can produce sonorous effects by a proper disposition 

 of carbon. The effect in these latter cases may be due either to 

 molecular pressure, in fact to a radiometer effect, though very 

 feeble in intensity ; or it may be the result of conduction through 

 the mass of the diaphragm, that is, radiant heat is reduced to 

 thermometric heat by absorption by the carbon deposit on the 

 outside of the disk ; and diis heat is transmitted through the disk 

 and radiated to the absorbent gases in the interior. 



Several experiments were made which fully establish the in- 

 ference that the effect is one of conduction, and that the black- 

 ened surface of an opaque body like zinc acts as though the 

 source of heat were transferred to the outside surface of the 

 disk. 



Tubes of various sizes and dimensions vzere now tried to 

 confirm Messrs. Bell and Tainter's observations on tubes. They 

 invariably gave out satisfactory sounds when the intermittent 

 rays were directed into the interior of the tubes, which were 

 always considerably intensified by blackening their interiors and 

 closing the open end with a glass plate. 



It was shown that there is a time element, and that the loud- 

 ness of the note emitted depends upon the rapidity with which 

 the contained air not only absorbs the degraded energy, but upon 

 the rapidity with which it gives up its heat to the sides of the 

 case and the exits open to it. Though the pitch of the maxi- 



mum note varied with the cavity and the amount of radiant heat 

 transmitted, its quality never varied, notwithstanding the great 

 diversity of materials used as diaphragms. 



Since these sonorous effects are due to the expansions of 

 absorbent gases under the influence of heat, and since wires are 

 heated by the transference of electric currents through them, it 

 seemed possilile that if we inclosed a spiral of fine platinum vrire 

 in a dark cavity, w ell blacked on the inside, and sent through it 

 by means of the wheel-break, rapid intermittent currents of 

 electricity from a battery, heat would be radiated, the air would 

 expand, and sounds would result. This was done, and the 

 sounds produced were excellent. In fact, with four bichromate 

 cells sounds more intense than any previously observed were 

 obtained . 



Furthermore it was evident that if the wheel-break were 

 replaced by a good microphone transmitter, articulate speech 

 should be heard. This was done, and an excellent telephone 

 receiver was the consequence, by means of which speech was 

 perfectly reproduced. 



The explanation of these remarkable phenomena is now 

 abundantly clear. -. 



It is purely an effect of radiant heat, and it is essentially one 

 due to the changes of volume in vapours and gases produced by 

 the degradation and absorption of this heat in a confined space. 

 The disks in Bell and Tainter's experiments must be diather- 

 manous, and the better their character in this respect the greater 

 the effect ; remove them, and the effect is gi-eater still. Messrs. 

 Bell and Tainter (Journal of Society of Telegraph Engineers, 

 December 8, 1880) showed that the sounds maintained their 

 timbre and pitch notwithstanding variation in the substance of 

 the disk, and M. Mercadier found that a split or cracked plate 

 acted as well as when it was whole. These facts are consistent 

 with the expansion of the contained air, but not with any 

 mechanical disturbance of the disks. Moreover M. Mercadier 

 showed that the effect was improved by lampblack, but he 

 applied it in the wrong place. 



The disks may, and perhaps do, under certain conditions, 

 vibrate, but this vibration is feeble and quite a secondary action. 

 The sides of the containing vessel must possess the power to 

 reduce the incident rays to thermometric heat, and impart it to 

 the vapour they confine, and the more their power in this respect, 

 as when blackened by carbon, the greater the effect. The back 

 of the disk may alone act in this respect. Cigars, chips of 

 wood, smoke, or any absorbent surfaces placed inside a closed 

 transparent vessel will, by first absorbing and then radiating 

 heat. rays to the confined gas, produce sonorous vibrations. 



The heat is dissipated in the energy of sonorous vibrations. 

 In all cases, time enters as an element, and the maximum effect 

 depends on the diathermancy of the exposed side of the cavity, 

 on its dimensions and surfaces, and on the absorbent character 

 of the contained gas. 



THE EARTHQUAKE IN ISCHIA 

 T^HE Island of Ischia, the Pithecusa of the ancients, is some 

 twenty miles in circumference, and appears to be the con- 

 tinuation of the north-western boundary of the Gulf of Naples 

 It consists of an old volcanic mountain sloping down on all side 

 to the sea. The southern rim of the old crater has been remove 

 by denudation, leaving the northern as a curved serrated ridge, 

 forming the peak of Monte Epomeo. 



Situated on the southern slopes of the island are only a few 

 and unimportant villages. 



Going from east to west along the northern slope we have 

 first the capital Ischia, then we encounter the great trachytic 

 lava stream which issues laterally from the slope of Epomeo, 

 and after a course of two miles entered the sea, forming a 

 promontory. This is the so-called lava Del' Arso, of a.d. 1302. 

 Next are encountered two very fresh-looking craters, from which 

 lava streams have flowed. Then we come to Casamicciola, a 

 small town of about 4000 inhabitants, to the north-west of which 

 is the village of Lacco Ameno. At the eastern end of the island 

 is the town of Forio. 



The district in which are situated Casamicciola and Lacco is 

 thus bounded on the north by the sea, on the south by the ridge 

 of Monte Epomeo, on the west by a spur stretching from the 

 latter into tlie sea, forming the Punta Coniacchia, and on the 

 east are the two hills called Monte Rotaro and Montagnone, the 

 new-looking craters already spoken of. 



