CHLADNI'S FIGURES. 1 95 



vibration there. The same experiment may be made by means of plates, 

 which will give us what are termed Chladni's figures. Suppose we strew a 

 glass-plate with fine sand, and stroke the edge with a fiddle-bow. The 

 vibrations of the plates will make certain patterns, and cast the sand upon 

 those points of repose to form nodal lines in various directions. The plates 

 must, of course, be held or fastened, and a variety of sound figures may be 

 produced. (See figs. 198 and 199.) 



The relation between the number of segments on the plate and the pitch 

 of the note, can be ascertained by using a circular plate clamped in the 

 centre. " If the finger on the plate and the fiddle-bow are one-eighth of 

 the circumference apart, the fundamental note will be produced. If one- 

 sixteenth apart, the higher octave will be heard." 



Sensitive flames will detect air vibrations, and flames can also be made 

 to sing. Sensitive flames were discovered by Mr. Barrett, who noticed the 

 effect a shrill note had upon a gas flame from a tapering jet. The flame 

 was a very long one (fourteen inches), and when the sound was produced it 

 shortened at once, while the upper part expanded like a fan ; the same 

 effects, in a less marked degree, were observable when the shrill sound was 

 prolonged from a distance of forty feet. Professor Tyndall was imme- 

 diately interested in this discovery, and in January 1867 he lectured upon 

 it at the Royal Institution. 



If any one wish to try the experiment, a piece of glass tubing should be 

 obtained, and let the mouth be tapered down to a small orifice one-sixteenth 

 of an inch in diameter. Then when the highest pressure is on for the 

 evening, light the gas and sound a shrill whistle. The flame will sink down 

 and spread out. The illuminating power may thus be increased, and many 

 experiments may be made. For instance, if a person be in the room and 

 try to read, he will probably not be able to do so at a little distance; but if 

 his friend whistle to the gas it will so expand itself as to enable him to read, 

 so long as the whistle lasts. 



A very ingenious burglar-detector was made upon the principle of the 

 sensitive flame, which expands at a noise and heats a welded plate of gold, 

 silver, and platinum. The plate swerves aside, the metals being unequally 

 affected by heat, and as it is connected with a battery, rings a bell by 

 electricity. A small high flame has been made sensitive to the chinking 

 of coin, or even to the ticking of a watch. We will now give some explana- 

 *ion, derived partly from Professor Tyndall, of the cause of sensitive flames. 



A sensitive flame is one just on the point of " roaring," and about to 

 change its aspect. " It stands," says Tyndall, " on the edge of a precipice. 

 The proper sound pushes it over . . . We bring it to the verge of falling, 

 and the sonorous pulses precipitate what was already imminent." The 

 flame is in a state of vibration, so sounds being vibrations, practically 

 increase the pressure ; and the flame acknowledges the pressure thus 

 invisibly applied by air waves. 



SINGING FLAMES are produced by burning hydrogen in a tube ; a 



