July 20, 1876] 



NATURE 



253 



SCIENCE IN GERMANY 

 {From a German Correspondent) 



IT is known to have been first discovered theoretically 

 by Maxwell, that the co-efficient of friction of a gas 

 is independent of the pressure. This law has been tested 

 and confirmed by Maxwell and O. E. Meyer, and more 

 recently by Kundt and Wz.rhVirg{Philosophical Mas^asine, 

 4, vol. iv. ; and fully in Poggendorff's Annalen, Bd. 155 

 and 156) with reference to the sliding of gases in limits, 

 between 760 and i mm. pressure of mercury. The latter 

 experimenters observed, as Maxwell did, the decrease of 

 vibrations of a round glass disc suspended bifilarly between 

 two fixed plates. At pressures under i mm. Kundt and 

 Warburg were unable accurately to investigate the friction. 

 They could perceive, however, that with continued pro- 

 gressive evacuation by the friction apparatus, the damping 



force exerted by the rarefied 

 gas on the motion of the 

 oscillating disc, decreases ; 

 still, even in the best vacuum 

 which could be produced, 

 it had still a considerable 

 value. Thus, e.g., in the best 

 hydrogen vacuum which 

 Kundt and Warburg could 

 produce, the damping force 

 was not less than one-third 

 of the value obtained with 

 full hydrogen-pressure (760 

 mm. mercury). 



To demonstrate the fric- 

 tion in such a vacuum before 

 a large audience, Prof. Kundt 

 recently constructed an ap- 

 paratus, which he employed 

 when giving a lecture on the 

 gas theory before a scientific 

 society in Berlin in March 

 last. The essential part of 

 the apparatus consists of 

 two small discs of mica, sus- 

 pended one over the other in 

 an evacuated space. When 

 the under disc, which, like 

 Crookes's radiometer, is fur- 

 nished with four light vanes 

 blackened on one side, is 

 set in rotation by the action 

 of light, the upper disc be- 

 gins to rotate in the same 

 direction (though much 

 more slowly) in conse- 

 quence of the friction of 

 some traces of air still pre- 

 sent in the apparatus. (The 

 upper disc of course no- 

 where touches the lower.) 

 The description of this ap- 

 paratus with drawings, will 

 shortly appear in Poggen- 

 dorff's Annalen. Here we 

 content ourselves with the 

 representation of a smaller 

 apparatus not meant for objective demonstration (see 

 annexed figure) which the eminent glass-artist, Geissler, 

 of Bonn, has constructed at the instance of Prof. 

 Kundt. Tnis apparatus, like the ordinary radiometer, 

 Is entirely inclosed in glass. On a fine steel point 

 rests, by means of a cap, the lower mica disc, with 

 the radiometer cross fixed to it. The upper disc rests 

 likewise on a fine steel point. This point is fixed to 

 an arm which reaches over the lower disc, but without 

 being in contact with it. On the upper disc a small mark 



is made (not shown in the figure), which enables one to 

 perceive whether the disc rotates or not. Illuminated by 

 the sun or candles, the radiometer cross with the mica 

 disc fastened to it, enters into quick rotation, and the 

 upper disc begins gradually to rotate in the same direction 

 as the lower one. S. W. 



ON MODES OF DEMONSTRATING THE 

 ACTION OF THE MEM BR AN A TYMPANI 



'T^HE movements of the bones of the tympanum in 

 •*■ connection with sound-waves were first observed 

 and their excursions measured by Buck (" Archiv. of 

 Ophthalmology and Otology," vol. i., 1870), and more 

 recently by Dr. Charles H. Burnett of Philadelphia, as 

 recorded in the same Journal for 1872. The method fol- 

 lowed in these researches was to expose the bones and 

 membrane by chiselling away a portion of the wall of the 

 tympanum, sprinkling on the chain of ossicles a little 

 powdered amylum, so as to secure bright vibrating points, 

 throwing I'ght into the cavity by means of a condenser, 

 and observing, with a microscope of low power, the ex- 

 cursions of the vibrating points when sound was conducted 

 into the external ear. Various interesting deductions 

 were drawn from these experiments as regards the 

 amplitude of the excursions of vibrating points on dif- 

 ferent portions of the conducting mechanism, and as to 

 the effect of variations of the fluid pressure within the 

 labyrinth on the extent of movement. In the last number 

 of the same journal an interesting paper appears from 

 Dr. Clarence J. Blake of Boston on " the use of the 

 Membrana Tympani as a Phonautograph and Logo- 

 graph," accompanied by a plate. Dr. Blake's method 

 consists of exposing the membrane and chain of bones, 

 and fixing a light style (made " by splitting long wheat 

 straw, scraping the inner cortical substance away, and 

 separating single fibres ") to the membrane. This style 

 is caused to record its movements on a plate of smoked 

 glass which is " carried smoothly and at a uniform speed 

 in a direction at a right angle to the direction of the 

 excursion of the style." ..." The membrana tympani 

 being set in vibration, and the carriage [bearing the 

 smoked glass] drawn by its weight, moving at right 

 angles to the excursions of the style, a wave-line, corre- 

 sponding to the character and pitch of the musical tone 

 sounded into the ear, is traced on the smoked glass." 



There is still a third method which I have recently 

 devised, chiefly for class illustration. It consists in con- 

 verting the tympanic cavity into a manometric capsule, 

 according to the method of Dr. Koenig {Philosophical 

 Magazine, 1873, vol. xlv. pp. 1-18, 105-114), and of view- 

 ing the oscillations caused by sound in a revolving mirror. 

 A preparation is easily made from the ear of the cat. 

 After sawing out the temporal bone, clearing away all 

 loose tissue, and exposing the tympanic bulla, I make two 

 small holes in the latter by means of a fine trephine. 

 Into these holes two glass tubes, of corresponding 

 diameter, are cemented with sealing wax— the one for 

 leading gas into the tympanic cavity, and the other out 

 of it. The preparation (which may be preserved in a moist 

 state in a well-stoppered jar for a long period, and be used 

 over and over again) is firmly fixed in a vice, one tube 

 is connected with the gas supply, and the other with one 

 of Kcenig's small burners. By means of a third tube 

 inserted into the external auditory meatus, sound-waves 

 are conducted to the membrane of the drum, the mirror 

 is rotated, and the usual pictures corresponding to the 

 pitch and quality of the sound are seen with exquisite 

 delicacy. I have found tones of medium pitch (ut 3 to 

 sol 4) produce the most distinct effects, and the vowels, if 

 uttered with sufficient intensity, produce pictures which 

 are quite characteristic. By using a preparation in which 

 the auditory apparatus on each side is present, it is easy 

 to devise an arrangement for showing the ;effects of 



