February 17, 1910J 



NATURE 



477 



saturation, and if k denotes tlie isometric specific lieat, 

 wliicli is constant or a function of the temperature only, 

 (i) s— K is always positive, increasing from R to 05 as 

 the reduced temperature t rises from o to i ; (ii) k-s' is 

 always positive, having a minimum value 4-96 R when 

 T=o72, and being co when t is either o or i ; (iii) in- 

 version in the sign of s' can thus occur when k/R>4g6 

 or ic = i + R/k<i-202, so that, on the assumption that 

 k/R = N + ^ for an N-atomic gas, inversion can occur only 

 if the gas has at least fiye atoms in its molecule ; (iv) the 

 latent heat of vaporisation increases continuously from o 

 to_ its largest value (27/8)RT as t falls from i to o, T 

 being the absolute critical temperature ; (v) the work of 

 vaporisation has a maximum value 055 RT when 

 T = o-7o. Clausius's characteristic similarly treated gives 

 widely different results :—(i) while s-k is always positive, 

 it is CO when t is either o or i, having a minimum value 

 15-3 R for T = o-83; ('') k — s' is always positive, is w 

 when T is either o or i, and has a minimum value 11-36 R 

 for T = o.8i; (iii) inversion in the sign of s' can thus 

 only occur if k/R>ii.36 or /f<i-o88, or, on the above 

 assumption, if there are at least eleven atoms in the 

 molecule ; (iv) the latent heat of vaporisation increases 

 contmuously from o to =c as t falls from i to o ; (v) the 

 work of vaporisation has a maximum value 069 RT when 

 T=o77. The contrast between these results is especially 

 marked for s-k and the latent heat.— Prof. Thornton: 

 The polarisation of dielectrics in a steady field of force. 

 E.xperiments on the polarisation of dielectric ellipsoids and 

 cyhnders suspended* in a steady electric field. From 

 measurements of the field-intensity, the dimensions of the 

 ellipsoids, and the frequency of torsional swings with and 

 \yithout the field, the dielectric constant can be found from 

 tmie to time. The longitudinal component of polarisation 

 reached a higher value than previously recorded, and was 

 found to be independent of the intensity of the field inside 

 the ellipsoid and to be quasi-elastic in type. Quartz, fused 

 and crystalline, flint-glass, amber, sulphur, ebonite, rubber, 

 gutta-percha, parafiin-wa.x, resin, and sealing-wax were 

 examined. From the rate of increase of the dielectric 

 constant the specific resistance of these was found by 

 considering the change of polarisation to be equivalent to 

 a current. The rate of depolarisation when the field was 

 reversed was the same as that of polarisation, and uniform 

 for several hours. The cause of this and for the in- 

 dependence of the field-intensity may be looked for in the 

 continued separation of molecular charge bv the attraction 

 of the opposite charges on adjacent molecules induced by 

 the application of the field. From a comparison of these 

 results and those in alternating fields, the variation of the 

 dielectric constants with frequency can be anticipated.— 

 A. Campbell : The use of mutual inductometers. In the 

 use of mutual inductometers, the use of a balancing coil 

 in one arm of the bridge causes considerable loss of 

 sensitivity. With an equal-arm bridge this difficulty is 

 overcome by putting the halves of the secondary circuit 

 m adjacent arms of the bridge. The auxiliary balancing 

 coil is dispensed with, and the usual formula i.s applicable. 

 The measurement of effective resistance, which is, in 

 general, more troublesome than that of self-inductance, 

 was discussed. The effective resistance determines the 

 total power spent by an alternating current in a conductor, 

 and is imnortant in telephonic and other high-frequency 

 work. When measured by a self-inductance bridge, large 

 errors may be introduced by the small residual inductances 

 pf the ratio arms. The analogous formulas for mutual 

 inductance bridges, which indicate that the inductances of 

 the ratio arms must be accurately proportional to their 

 resistances if errors are to be avoided, are here worked out 



and N, are the numbers of turns in the windings of the 

 ring. The method is applicable to the testing of current 

 transformers. 



Mineralogical Society, January 25.— Prof. W. J. Lewis, 

 F.R.S., president, in the chair. — Dr. S. J. Shand : A 

 group of minerals formed by the combustion of pyritous 

 shales in Midlothian. At the Emily coal-pit, Arnist'on, as 

 the result of the slow combustion of a heap of shaly refuse, 

 which became spontaneously ignited, presumably owing to 

 the evolution of heat caused by the atmospheric oxidation 

 of pyrites, a number of uncommon mineral species have 

 been formed, of which five have been recognised, viz. 

 native sulphur, sal-ammoniac, tschermigite, mascagnite, 

 and a possibly new species, aluminium sulphate. — ^^Prof. 

 W. J. Lewis : A crystal-holder for measuring large speci- 

 mens. For this purpose a clamp of convenient form and 

 with various adjustments has been designed and made by 

 Mr. Pye. — T. Crook : Some observations on pleochroism. 

 The phenomena of pleochroism displayed by plates of 

 coloured minerals when examined in ordinary light were 

 treated in a general way for both parallel and convergent 

 rays, and the factors upon which they depend were dis- 

 cussed. — L. J. Spencer : Notes on the weight of the 

 " Cullinan " diamond, and on the value of the carat- 

 weight. Varying statements of the weight of the 

 " Cullinan " diamond, in its original, uncut form have 

 been published, but from a comparison of the carat-weights 

 against which it was weighed in 1905 it is concluded that 

 the correct weight was 621-2 grams, or 3025? English 

 carats of 205-304 milligrams (as defined by the Standards 

 Department of the Board of Trade in 1S89). Other values 

 are, however, given for the English carat and for the carat 

 in other countries, and the average value has decreased, on 

 the whole, in course of time. The carat-weight had its 

 origin in the use as weights of seeds of Ceratonia siliqua, 

 which weigh approximately a carat. The existing con- 

 fusion would be obviated by the general adoption of the 

 metric carat of 200 milligrams (one-fifth of a gram) recently 

 recommended by the International Committee of Weights 

 and Measures (Nature, igo8, vol. Ixxii., p. 611). — Dr. 

 G. T. Prior : A basalt from Rathjordan, Co. Limerick. 

 .Specimens of basalt from Rathjordan in the Allport collec- 

 tion in the British Museum show in thin slices under the 

 microscope round sections of isotropic material containing 

 central and marginal inclusions, and thus resembling small 

 leucites. The rock is very similar, mineralogically and 

 chemically, to leucite-basalts from Bohemia, but contains 

 only a small fractional percentage of potash. This fact, 

 combined with observations of the refr.active indices, leads 

 to the conclusion that the isotropic material is mainly 

 analcite, and not leucite. — Dr. G. F. H. Smith and Dr. 

 G. T. Prior : .A. fluo-arsenate from the Indian manganese 

 deposits. A crvstallographical and chemical examination 

 made of the green arsenate from Kajlidongri, Jhdbua 

 State, mentioned in Mr. Fermor's monograph on the 

 manganese-ore deposits of India (Rec. Geol. Surv. India, 

 iqo8), led to the following results : — composition, 

 (.MgF)Ca.'\sO, ; specific gravity, 3.768 ; hardness. 3J ; 

 colour, apple- to brownish-green; monoclinic, a:b:c = 

 °'74S5 -J : "■'"'453. 5=120° 50'; forms present, (010), (no), 

 (ill), (ml, (3ii\ (112V IT:;-}''; good cleavage parallel to 

 (Toi), and partings parallel to (no), (foa), (331); twin 

 plane, (100) ; refractive indices, 1-640, 1-660, 1-666 : acute 

 bisectrix nearly perpendicular to (Toi), and axial plane at 

 right angles to the plane of symmetry, but no horizontal 

 dispersion was noticed: 2E=io5° approximately, with 

 negative birefringence. The material is probably identical 

 with tilasite, which was first described by Sjogren in 1905 

 from the manganese deposits of Liingban. 



A null method in iron testing anaIogouVto^Ma"xlvi;n'; U-'i""f Clark^^^^n^P-nf"'??''!' n' l^=>ngban Sweden.- 

 self-inductance method is described. The ring to be tested ^'ti^ ^-'"^'"'^ ^"*^ ^'°'- "' ^- Bowman: The composi 

 is_ wound with primary and secondary coils. The magnet- 

 ising current, I,, is passed through the primary coil, the 

 primary circuit of a mutual inductometer, and a slide-wire 

 resistance. The detecting instrument is put across a 

 circuit consisting of the secondaries of the ring and the 

 mductometer in ooposition, and a part, Q, of the slide- 

 wire_ resistance. By .adjusting Q and the reading M of 

 the inductometer a balance is obtained, in which case the 

 power lost in the ring is equal to Q1,-xf^ ,/'!<.., where N, 

 NO. 2103, VOL. 82] 



on of a stone from the meteoric shower which fell at 

 Dokdchi, Bengal, on October 22, 1903. The small crusted 

 stone examined, weighing 17-8 grams, shows chondritic 

 structure, and belongs to the class Ci of Tschermak. The 

 chief constituent minerals are bronzite (37-9 per cent.), 

 olivine (37-7 per cent.), nickel-iron (18.5 per cent.), troilite 

 (4-1 per cent.). — Dr. G. F. H. Smith exhibited cut and 

 rough specimens of synthetical sapphire recently produced 

 by Prof. Verneuil, oxides of iron and titanium being the 

 colouring agents. 



