M. A. Kundt's Acoustic Experiments. 45 



These dust-figures furnish a convenient means for determining 

 the velocity of sound in solids and in gases. 



Since the velocity of sound in air is known for a given tempe- 

 rature, from the distance of corresponding places of two succes- 

 sive heaps of dust, which is equal to half the length of the sound- 

 wave, we get the number of vibrations of the tone of the column 

 of air, and, as this tone is the same as that of the tube, the num- 

 ber of vibrations of the material of the tube. Conversely, from 

 the known velocity of sound in the material of the tube, the ve- 

 locity of sound in an enclosed gas may be calculated from the 

 observed length of the enclosed gas. In tubes in which the ve- 

 locity of sound is equal, and which are filled with different gases 

 and held in the middle wdiile being rubbed, the velocities of 

 sound in the gases are inversely as the numbers of heaps of dust. 

 This mode of determination is sufficiently accurate. But the 

 desire to get still more accurate results, and to be able to deter- 

 mine the velocity of sound in solids, led M. Kundt to produce the 

 dust-figures, not in the sounding tube, but in an adjacent column 

 of air. Over the closed end of a glass tube a few feet in length, 

 a somewhat wider glass tube was pushed to a distance of a quarter 

 the length of this closed tube, and was here firmly fixed by a 

 cork. At the other end of the outside tube was a cork, which 

 by means of a rod could be moved backwards and forwards. If 

 now the first tube, by which the apparatus is held in the hand, 

 be rubbed in the middle so that two nodes are formed, the vibra- 

 tions of the closed end set in motion the air of the tube sur- 

 rounding it, and produce a series of dust-figures the lengths of 

 which are to the length of the rubbed tube as the velocity of 

 sound in the gas is to that of the material of the tube. If instead 

 of the first tube any rod be taken which can excite sound, the 

 quotients of the lengths of the sounding body by the distances 

 of corresponding points of successive divisions of the dust-figures 

 are as the velocity of sound in the sounding body. The figures 

 thus obtained are far sharper than those in the sounding tubes, 

 and therefore permit a more accurate measurement. According 

 to the ratio of the length of the sounding column of air to 

 the semiwave-length, the dust-figures may materially differ in 

 form. If, while the inner tube is sounding, the cork at the 

 end of the outer be moved backwards or forwards, a place is soon 

 found at which the whole of the lycopodium in the tube colle cts 

 in small heaps at equal distances from each other, the extreme 

 heap just touching the moveable cork. Each of the heaps is at 

 an air-node, and the space from the moveable cork to the end of 

 the sounding tube is divided by them (when they are formed) 

 into a number of exactly equal parts, so that the length of the 

 column of air is the exact multiple of half a wave-length. If 



