258 



NATURE 



I'/tily 15, 1 8^0 



Deprez. — An experiment in physiological optic=, by M. Bibart. 

 —Measurement of the refractive indices of liquids, by MM. Mace 

 de Lepinay. 



SOCIETIES AND ACADEMIES 



London 



Royal Society, May 27. — "A Preliminary Account of the 

 Reduction of Observations on Strained Materials, Leyden Jars, 

 and Voltameters," by John Perry and W. E. Ayrton. Commu- 

 nicated by Prof. G. G. Stokes, Sec. R.S. [Abstract]. 



In discussing the residual-charge phenomena of condensers, 

 the authors point out that in spite of certain elaborate measure- 

 ments which have been made on different kinds of glas', nobody 

 has yet discovered a constant such that it expresses the residual 

 charge property of a particular substance. They therefore say 

 that the simple plan of charging a Leyden jar for a long time, 

 short-circuiting for a small definite period, then insulating and 

 giving the residual charge at certain times from insulation (thus 

 getting say three definite numbers for each dielectric experi- 

 mented upon), is more accurate than, and is ju-t as definite as, 

 any plan hitherto proposed for determining the residual-charge 

 properties of a dielectric. They show that if Prof. Clerk 

 Maxwell is right, the only correct means of studying these 

 properties are given by the constants of Maxwell's differential 

 equation, and they describe experiments on the Leyden jar of a 

 Thomson's electrometer, and reductions of observations to obtain 

 such constants. Thus one such constant is found to satisfy all the 

 observations made from the 500th to the 900th minute of insula- 

 tion of a jar. The authors draw attention to the analogy which 

 they have pointed out between condensers and voltameters 

 charged by electromotive forces less than one and a half volts, 

 and show that if we assume Maxwell's equation to be true for 

 voltameters, that is, if we assunr.e a voltameter to be a condenser, 

 one constant satisfies observations from the 50th to the 190th 

 minute of charging, and from the 20th to the 80th minute of 

 discharging. They then proceed to develop a theory of the 

 increasing strains in bodies subjected to constant stresses. When 

 a homogeneous substance is suddenly subjected to stress, there is 

 a suddenly produced strain which follows Hooke's law, depend- 

 ing on a constant k, but besides this there is a viscid increase of 

 strain whose rate is proportional to the stress depending on a 

 constant r. In steel the viscous strain is not of much importance, 

 whereas in water strained by bodies moving in it it is very 

 important, as it is also when a beam of sealing wax is 

 loaded. They show that the viscid increase of strain is exactly 

 analogous with the flow of electricity in accordance with Ohm's 

 law, and that the suddenly-produced strain is analogous with in- 

 duction ; and considering a heterogeneous material subjected to 

 shearing stress, they find that the above assumptions lead, for 

 strained materials, to exactly the same equation as Prof. Maxwell 

 found for condensers. They found that the support of this 

 theory is exactly the same as the support which they have given 

 of Maxwell's theory of condensers. Thus one constant of the 

 equation satisfied the recovery from deflection of a glass beam 

 from the 4th to the 240th hour of relief, and satisfied the re- 

 covery from twisting of a glass fibre for all but the first few 

 observations. They have also constructed a voltameter such 

 that the platinum electrodes may be maintained at any tempera- 

 ture in an atmosphere of any gas for any length of time, main- 

 taining a vacuum over the liquid or saturating it w ith any gas, 

 and they give the different values of the residual charge constant, 

 which satisfies all but the first few observations of charge and 

 discharge in diflerent cases. The authors conclude their paper 

 by saying that, regarding a voltameter as a condenser, then as 

 the plates of the charging battery are larger and nearer together, 

 and as the times of charge and discharge of the voltameter are 

 made less and less, the more do the total quantities of the charge 

 and discharge approximate to one another. 



Physical Society, June 26. — Prof. W. G. Adams in the 

 chair. — Mr. C. V. Boys read a paper, by Prof. Guthrie and him- 

 self, on the measurement of the conductivity of li(iuids by means 

 of magneto-electric induction. The liquid is suspended in a 

 glass vessel by a fine iron wire in the centre of a cylindrical 

 electro-magnet formed of two semicircular parts. This electro- 

 magnet is rotated at a velocity not exceeding 3,000 turns per 

 minute, and the liquid being drawn round in the direction of 

 rotation, the wire is subjected to torsion, which, under correction 

 for certain errors, is proportional to the resistance of the liquid. 

 The torsion is observed by means of a scale and microscope^ 



The results, plotted in a curve, agree very closely with those of 

 Kohlrausch, obtained by alternate currents, and Dr. Guthrie 

 thinks that they are probably more correct and trustworthy than 

 Kohlrausch's, for the method w ould seem to be superior and the 

 curve contains fewer excentric points than his. — Dr. Gladstone 

 read a paper on the refraction equivalents of isomeric bodies, in 

 which he described the present state of the subject and his own 

 contributions to it. He showed that the refractive power of 

 bodies ovec light was of great importance to chemists, since it 

 depended on their es ential structure. — Dr. Huggins described 

 his latest results of star spectra, and illustrated his remarks by 

 photographic spectra taken by his improved method. From 

 these it appears certain stars, such as Vega, give a complete 

 spectrum of hydrogen. Others, more yellowish in colour, show 

 a thinning of these lines, such as Sirius, tj Ur^e Majoris. Others 

 show theintrusion of more refrangible lines ; forexample, Arcturus, 

 a Aquila, o Virginius ; while Capella gives a complex spectrum 

 like that of the sun. Dr. Huggins also showed a spectrum of 

 the flame of a spirit lamp, which presented a strong group of 

 lines atS, and he considered it to repre-ent the light emitted by 

 the molecules of water. He further observed that the spectrum 

 offered a highly sensitive test of the presence of carbon. — Mr. 

 Liveing exhibited a new fire-damp indicator, capable of detecting 

 I per cent, of marsh g.as in air. It is based on the fact that an 

 incandescent body shows more brilliantly in proportion to the 

 amount of marsh gas in the air, and consists of two fine platinum 

 w ires kept incandescent by a magneto-electric current sent 

 through them in one circuit. One wire is excluded from the fire- 

 damp, the other is exposed to it, and the relative intensities of 

 the two glowing wires is compared by a photometric screen 

 placed between them and adjustable to a position between them 

 at which the reflections of the w'ires on the screen are of equal 

 intensity. The position of the screen relatively to the wires is 

 given by a scale, and measures the propoi tion of fire-damp in 

 the air. This contrivance is more advantageous than the safety- 

 lamp, which only indicates 2 per cent of marsh gas in the air. — 

 Dr. .Stone exhibited a vacuum-tube of vari.able resistance and a 

 large electro-magnet wound with iron wire. The former consists 

 of a barometer-tube thirty-two inches long, terminating above 

 in a short vacuum- chamber arranged transversely, and closed at 

 either end by adjustable india-rubber stO|>pers, through which 

 platinum terminals are passed. Above this the vertical tube is 

 continued to a glass stopcock, by means of which small quanti- 

 ties of air can be introduced. The foot of the tube is attached 

 to an india-rubber flexible pipe with a cistern like that of Frank- 

 land's gas apparatus. The cistern full of mercury is counter- 

 balanced, and can be raised or lowered at will through the whole 

 thirty-two inches. A Torricellian vacuum can thus be made in 

 the upper chamber, or one of more or less perfectness. On 

 passing the induction-spark between the terminals in the former 

 case all the di-charge is carried off, none appearing at the dis- 

 charger. By gradually raising and lowering the cistern, after 

 admitting a little air by the stopcock, the resistance of the partial 

 vacuum thus obtained can be altered within wide limits. A point 

 can also be found where the spark of breaking-contact is shunted 

 through the vacuum-tube, while the weaker discharge of making- 

 contact is stopped. The induction-current is thus obtained in a 

 single direction, a matter of some importance in physiological 

 experiments. The electro-magnet could not be described from 

 pressure of other matter. Its peculiarities consisted in its being 

 wound with best charcoal-annealed wire of about 5 millim. sec- 

 tion in four parallel circuits, and in each pole being cast, after 

 winding into a solid block of paraffin. It was exi ected that the 

 latter device would increase the inductive effect of the spirals ; 

 and indeed it appeared that the lifting power was somewhat 

 strengthened. The cores had been originally wound with large 

 copper wire of about the same weight as the iron wire. But the 

 lifting power for batteries of moderate size, five or six Bunsen's 

 cells, for instance, had increased fourfold after the substitution. — 

 A paper by Mr. McFarlane Gray entitled specific heats calcu- 

 lated from entropy. This is a re affirmation of a paper on the 

 v.ilue of V, decUned by the committee of the Royal Society in 

 February, 1878. The author read a paper at the last meeting of 

 the Institution of Naval Architects, which we said was a 

 singularly bold and original attempt to account for many of the 

 phenomena of steam and other effects of heat when applied to 

 matter. In the present paper Mr. Gray continues in the same 

 line of startling generalisation. The following is a specimen :— 

 Taking ihepv of hydrogen at 493°'2 F., as in Rankine's tables, 

 to be 378819 foot pounds, he writes — 



