Jtme^,, 1875] 



NATURE 



89 



from the under one, it was necessary to place a certain I 

 over- weight into the other scale of the balance. 



It was found that the separation of the two plates can j 

 be accomplished by any force, however small, only the : 

 time in which the distance of the plates is increased by a 

 certain fraction through the action of such a force, is all 

 the greater, the smaller this force is. This time is still ; 

 greater if the two plates are in water or in another liquid, 

 instead of in air. To give an idea of this we may mention ; 

 that the distance of two plates, of 155 millimetres dia- i 

 meter, under water, which originally was ci mm., was \ 

 increased in consequence of the continuous pull of 

 I gramme by o'oi mm. only in i^ minutes, by o'l mm. \ 

 only in 7 minutes. j 



Herr Stefan in his experiments measured the time that | 

 passed while the original distance of the plates increased ; 

 by a certain fraction. First, the law was established for 

 the motion of the plates in liquids as well as in air, that 

 the times stand in the reverse proportion to the separating 

 force. With the same overweight they are the longer, 

 the smaller the original distance of the plates, but this in 

 a far greater than a simple proportion ; they increase 

 nearly in square proportion if the distance of plates de- 

 creases in a simple one. For different sized plates the | 

 times in question stand in the proportion of the fourth • 

 powers of the semi-diameters of the plates ; for different 

 liquids in the same proportions as the times which elapse, i 

 while equal volumes of these liquids flow through a capil- ; 

 lary tube, under equal pressure. j 



It results clearly that with this phenomenon there rests 

 a problem of hydrodynamics and not of molecular forces. 

 The phenomenon can be explained in the following 

 manner : — When the separating force begins to act, the 

 distance of the plates is increased by an infinitely small 

 part. The space contained between the plates is thus 



enlarged, the liquid therein contained is dilated, and con- 

 sequently its hydrostatic pressure decreased. The over- 

 pressure of the exterior liquid acts against the separating 

 force. No equilibrium is, however, attained, because the 

 decrease of hydrostatic pressure between the plates causes 

 an inflow of the exterior liquid and thus a decrease of 

 the difference of pressure. The distance of plates may 

 be again increased by the separating force, and then the 

 same process is repeated in a continuous manner. 



Herr Stefan has therefore given the name of apparent 

 adhesion to this phenomenon. He has tried to deduce 

 theoretically all the different laws to which the different 

 experiments have led him ; he has succeeded in finding 

 an equation which expresses these laws, and which at the 

 same time permits the deduction of the co-efficients of 

 interior friction of the liquids experimented with, directly 

 from the experiments. The values of the coefficients 

 obtained in this manner correspond almost exactly with 

 those obtained by the experiments of Poisseville, Maxwell, 

 and O. E. Meyer. But as Herr Stefan thinks the theore- 

 tical solution of the problem only an approximate one, 

 we reserve further details on the subject. 



If we rub a wet cloth quickly over a glass tube, 

 closed at both ends, it is caused to vibrate longi- 

 tudinally. If at the same time it gives its lowest 

 longitudinal note (as we will suppose for the sake of 

 simplicity), then the end planes of the tube strike quite 

 periodically against the air enclosed in it, and cause the 

 same to vibrate. These vibrations are isochronous with 

 those of the tube itself They proceed from both ends of 

 the tube towards one another, and, as a consequence, 

 standing waves are formed in the enclosed air column. 

 If into such a tube lycopodium or silicic acid has been 

 placed, these powders (as also Herr Kundt has discovered 



ik 



*, 



a few years ago) collect at the node points of the standing 

 waves and form figures of a very peculiar kind. As the 

 length of these standing waves depends solely on the 

 height of the generating sound and of the velocity of the 

 waves in the gas, with which the tube is filled, the pro- 

 portion of this wave-length to the wave-length in the 

 glass gives the relative velocity of sound in air, with that 

 in the glass as unity. Herren Kundt and Lehmann at 

 Strasburg have lately tried to produce longitudinal 

 vibrations and the figures just mentioned in a liquid, 

 enclosed in a cylindrical tube, in a similar manner. 

 It was found that in a column of water standing waves 

 and figures can be produced almost as easily as in a 

 column of air. The apparatus which was used for this 

 purpose consisted of a glass tube, A B, closed at one end, 

 B, which was placed firmly into a wider glass tube, C D, 

 by means of an india-rubber stopper. The latter glass 

 tube was closed at end D, and had two lateral outlets with 

 stopcocks, so as to be easily filled with water. The 

 powder which is placed in the tube C D must be suffi- 

 ciently heavy and of a certain degree of fineness ; it is 

 best to use for this purpose finely divided iron [Fa-rum 

 limaUim). The column of liquid must be free of even 

 the smallest air-bubble. If the liquid used, for instance 

 water, contains a gas absorbed, it must be first freed 

 from it by continual boiling. In order to make the 

 apparatus sound it is necessary only to rub a wet cloth 

 quickly over the protruding part of the tube A B. 



The figures in this column of liquid may serve for the 

 determination of the velocity of sound in the liquid. If 

 the end A of the sounding tube is closed by a cork, and if 

 then over this end another tube is attached, which contains 

 lycopodium, then, by the figures which occur in the hquid, 

 and by those which occur in the tube with air, the wave- 

 length of the same sound is obtained both in liquid and 



in air. The proportion of both gives the relative velocity 

 of sound in the liquid with reference to that in air as 

 unity. This relative velocity multiplied by the absolute 

 velocity in air at the same temperature, gives the absolute 

 velocity of sound in the liquid at the temperature in 

 question. It was interesting to compare the results of this 

 method of determining the velocity of sound in water, 

 with the values required by the ordinary theory of the 

 velocity of sound. According to the theory based on the 

 experimentally determined elasticity of water, the velocity 

 of sound at 8° Celsius is 1,437 metres. Colladon and 

 Sturm, by their experiments in the Lake of Geneva, 

 found the same to be 1,435 metres at 8° C. Although the 

 remarkable coincidence of these values is only acci- 

 dental, it is nevertheless proved that experiments such as 

 those of Colladon and Sturm do not give figures that are 

 very far from the theoretical values. The experiments of 

 Kundt and Lehmann show that the diameter and thickness 

 of the glass of the tube, which is used for the determina- 

 tion of the velocity of sound according to the method 

 above described, greatly affect the value of the velo- 

 city of sound in water. In a tube of 2*2 mm. thick- 

 ness of side, and 287 mm. diameter, the velocity at 

 18° C. was io4o'4 metres (the mean of two experiments 

 which coincided very closely) ; in another one of 5 mm. 

 thickness of side and 14 mm. diameter, the velocity was 

 found I382-2 metres at 22*2° C. As it would be very diffi- 

 cult to avoid unevenness in the sides of the tube, it does 

 not seem probable that when using tubes the above value 

 of 1,435 nietres could be completely reached. These 

 experiments, proving the influence of the thickness of the 

 sides and diameter of a tube upon the velocity of sound 

 in water, contradict the hypothesis of Wertheim, accord- 

 ing to which a column of liquid, which is sounding or 

 conducting sound, benaves like a firm rod. S.W, 



