44 M. A. Kundt J s Acoustic Experiments. 



closed by well-fitting corks, the dust, which has previously been 

 uniformly distributed, collects no longer in those nodes of the 

 glass, but forms a series of heaps, each of which consists of a 

 number of fine cross lines. If the tube be again rubbed, the 

 dust again whirls about, and, when the tone sounds, deposits 

 again in the same manner. But if the sound be not allowed to 

 die out slowly, but be suddenly interrupted, by rapidly rubbing 

 the tube with a quick stroke and then suddenly clasping it with 

 the cloth in the middle of a stroke, the same periodical accumula- 

 tions of dust are observed; but the delicate ripples have disap- 

 peared, and the whole appearance of the figure is more indistinct. 

 These accumulations are due to permanent vibrations into which 

 the column of air is put by the oscillations of the ends of the 

 tubes, and which are isochronous with these oscillations. Each 

 accumulation corresponds to half a wave-length. Sound travels 

 in glass a little less than sixteen times as rapidly as in air. If, 

 therefore, a tube be held in the middle while it is being rubbed, 

 so that its length forms half a wave-length, there are sixteen ac- 

 cumulations of the lycopodium powder. If two corks are placed 

 in the tube at a distance from the two ends of a quarter the 

 length, and the tube be allowed to give the second longitudinal 

 tone, the corks, since they are at the nodes, remain at rest, and 

 the column of air they enclose is as little in vibration as is the 

 air in a tube open at both ends. No dust-figures are then 

 formed ; but they immediately appear if one of the corks be again 

 brought to the end of the tube, or placed between the two 

 nodes. Also if there is only one cork at any vibrating part of 

 the tube, dust-figures are obtained ; yet the motion of the air is 

 most energetic if both ends are closed. To produce dust-figures, 

 it is unnecessary that the column of air be a whole multiple of 

 half a wave-length ; yet the ease with which the figures are pro- 

 duced, as well as their form and regularity, appears to depend on 

 the ratio of the length of the sound-wave to that of the column 

 of air. 



The following is adduced as a characteristic property of the 

 figures of frequent occurrence. Let a tube in which lycopodium 

 powder is as uniformly distributed as possible, be rubbed with a 

 stroke w r hich is not too powerful. Accumulations of dust are 

 then met w r ith, very beautifully formed of ripples; between every 

 two of these accumulations there is a circular or elliptical space 

 which is tolerably free from dust, and is surrounded by a delicate 

 ring of dust. These rings or holes are largest at the nodes of 

 the longitudinal motion of the tube, and diminish in size towards 

 the middle between two nodal points. 



Sand or other heavy bodies are not moved by the waves of air 

 in the sounding tubes. 



