,58 



NA TURE 



[February i i. i8q2 



were glasses (one with spherulites), the fifth exhibited skeletal 

 crystals of felspar with a peculiar grouping, rarely and imperfectly 

 seen in naturally-cooled basalts. With these were compared 

 two specimens of magma-basalts, obtained by the author from 

 the Rowley mass, which exhibited a very different structure. 

 The author suggested that this difference might be due to the 

 absence of water from the artificially melted rock, which might 

 also account for the rarity of tachylites in nature. 



February 4. — "On the Mechanical Stretching of Liquids: 

 an Experimental Determination of the Volume-Extensibility of 

 Ethyl Alcohol." By A. M. Worthington, M.A. Communi- 

 cated by Prof. Poynting, F.R.S. 



After adverting to the three known methods of subjecting a 

 liquid to tension, viz. (i.) the method of the inverted barometer, 

 (ii.) the centrifugal method devised by Osborne- Reynolds, (iii.) 

 the method of cooling discovered in 1850 by Berthelot, and 

 poiriting out that the first two afford means of measuring stress 

 but not strain, while the third gives a measure of strain but not 

 stress, the author proceeds to describe the manner in which he 

 had used the method of Berthelot in combination with a new 

 mode of determining the stress, and had succeeded in obtaining 

 simultaneous measures of tensile stress and strain for ethyl 

 alcohol up to a tension of more than 17 atmospheres, or 255 

 pounds per square inch. 



The liquid, deprived of air by prolonged boiling, is sealed in 

 a strong glass vessel, which it almost fills at a particular tem- 

 perature, the residual space being occupied only by vapour. 

 On raising the temperature, the liquid expands and fills the 

 whole. On now lowering the temperature, the liquid is pre- 

 vented from contracting by its adhesion to the walls of the 

 vessels, and remains distended, still filling the whole and exert- 

 ing an inward pull on the walls of the vessel. The tension 

 exerted is measured by means of the change in capacity of the 

 ellipsoidal bulb of a thermometer sealed into the vessel and 

 called the "tonometer." This bulb becomes slightly more 

 spherical, and therefore more capacious, under the pull of the 

 liquid, and the mercury in the tonometer-stem falls. The 

 tension corresponding to the fall is previously determined from 

 observation of the rise produced by an equal pressure applied 

 over the same surface. 



The liquid is caused at any desired instant to let go its hold 

 and spring back to the unstretched volume corresponding to its 

 temperature and to its saturated vapour pressure, by heating for 

 a moment, by means of an electric current, a fine platinum 

 wire passing transversely through the capillary tube that forms 

 part of the vessel. The space left vacant in the tube represents 

 the apparent extension uncorrected for the yielding of the glass 

 vessel. 



The measures obtained show that, within the limits of 

 observational error, the stress and this apparent strain are 

 proportional up to the highest tension reached (17 atmo- 

 spheres) ; but, since the small yielding of the nearly rigid glass 

 vessel must itself be proportional to the stress, it follows that 

 the stress and absolute strain are proportional. 



By subjecting the liquid to a pressure of twelve atmospheres 

 in the same vessel, it was found that the apparent compressibility 

 was the same as the apparent extensibility, whence it is deduced 

 that between pressures of + 12 and — 17 atmospheres the 

 absolute coefficient of elasticity is, within the limits of observa- 

 tional error, constant. Its actual value is best obtained by 

 observations of compressibility. 



The paper concludes with a description and explanation 

 of a peculiar phenomenon of adhesion between two solids 

 in contact when immersed in a liquid that is subjected to 

 tension. 



Physical Society, January 22. — Prof. O. J. Lodge, F.R.S., 

 Vice-President, in the chair. — Prof. G. F. Fitzgerald, F.R.S., 

 read a paper on the driving of electromagnetic vibrations by 

 electromagnetic and electrostatic engines. The author pointed 

 out that as the electromagnetic vibrations set up by Leyden 

 jar or condenser discharges die out very rapidly, it was very 

 desirable to obtain some means whereby the vibrations could be 

 maintained continuously. Comparing such vibrations with those 

 of sound, he said the jar discharges were analogous to the tran- 

 sient sound produced by suddenly taking a cork out of a bottle ; 

 what was now required was to obtain a continuous electromag- 

 netic vibration analogous to the sound produced by blowing 

 across the top of a bottle-neck. In other words, some form of 

 electric whistle or organ-pipe was required. These considera- 



NO. II 63, VOL. 45] 



tions led him to try whether electromagnetic vibrations could 

 be maintained by using a discharging circuit part of which was 

 divided into two branches, and placing between these branches 

 a secondary circuit turned to respond to the primary discharge. 

 This did not prove successful, on account of thei-e being nothing 

 analogous to the eddies produced near an organ-pipe slit. The 

 analogy could, he thought, be made more complete by utilizing 

 the magnetic force of the secondary to. direct the primary cur- 

 rent first into one of the two branches and then into the other. 

 If spark gaps be put between two adjacent ends of the branches 

 and the main wire, then the magnetic effect of the secondary 

 current should cause the spark to take the two possible paths 

 alternately. Electrically-driven tuning-forks and vibrating 

 spirals were cases in which magnetic forces set up vibrations, 

 but here the frequency depended on the properties of matter, 

 and not on electrical resonance. The frequency of delicate 

 reeds could, however, be controlled by resonance cavities with 

 which they were connected, and he saw no reason why the same 

 action could not be imitated electromagnetically, usmg an elec- 

 tric spark as the reed. Referring to the properties of iron in 

 connection with electromagnetic vibrations, he pointed out that 

 a prism of steel i millimetre long had a period of longitudinal 

 vibration of about one-millionth of a second, and, as this was 

 comparable with the rates of electromagnetic vibrations, the 

 immense damping effect which iron had on such vibrations 

 might be due to the setting up of sound vibrations in the mate- 

 rial. Other methods of driving electromagnetic vibrations had 

 suggested themselves in the shape of series dynamos or alter- 

 nators. The polarity of a series dynamo driving a magnetic 

 motor would, under certain circumstances, reverse periodically, 

 and thus set up an oscillatory current in the circuit. Similar 

 effects can be got from series dynamos charging cells or con- 

 densers. In an experiment made two weeks before, with Plante 

 cells and a Gramme dynamo, reversals occurred every fifteen 

 seconds. Greater frequencies might be expected with con- 

 densers. The latter case he had worked out theoretically. 

 He had also tried experiments with Leyden jars and a dynamo, 

 but got no result. This might have been expected, for the cal- 

 culated frequency was such as would prevent the currents and 

 the magnetism penetrating more than skin deep. Calling the 

 quantity of electricity on the condenser Q, the differential equa- 

 tion for a dynamo of inductance L, and resistance r, and a 

 condenser of capacity x is 



LQ + rQ + 2 



LQ+(' 



LQ. 

 L)Q -f 5 = o. 



If L be = o, the solution of the equation is 



Q = Qo6 L cos27rA, 



and the rate of degradation of amplitude depends on the factor 



If, however, L be greater than r, the exponent of e becomes -f , 

 and hence Q would go on increasing until limited by the satura- 

 tion of the iron or the increased resistance of the conductors 

 due to heating. A dynamo without iron, provided one could 

 be made to run fast enough to send a current through itself, 

 would be likely to give the desired effect. The author thought 

 that by making such a dynamo large enough and its armature 

 very long, it would be possible to get a frequency of about one 

 million. Electrostatic machines seem, however, to be more 

 promising driving agents. Like series dynamos, their polaiity 

 depends on the initial charge, and can be easily reversed. 

 Hitherto such machines have been inefficient mainly on account 

 of the sparking in them, but Maxwell had shown how this 

 could be obviated. There was the same kind of difference 

 between electromagnetic and electrostatic machines as between 

 Hero's engine and the modern pressure engine. Like modern 

 engines electrostatic machines worked by varying capacity, but 

 the effect of this variation in electrostatic machineswas only to 

 vary the frequency and not the rate of degradation. From the 

 fact that electrostatic multipliers could be driven by alternating 

 currents, he thought they might be made to drive alternating 

 currents. If magnetic currents could be obtained, then electro- 

 static engines would easily be produced. In conclusion, the 

 author described a modified electrostatic rqultiplier which he 



