548 



PHYSICS, PROGRESS OP, IN 1902. 



nolds and Moorby obtained by direct mechanical 

 means. The difference is ascribed to the electro- 

 motive force of the Clark cell, to which the elec- 

 trical measurements were referred. 



Relation between Temperature and Pressure. 

 According to the hypotheses of Poynting and Ost- 

 wald, this relation for a two-phase one compo- 

 nent solid-liquid system varies according as the 

 melt can or can not flow freely away. Thus, ac- 

 cording to these views, ice will melt, provided the 

 water can escape, at much lower pressures than 

 those corresponding to its ordinary pressure-tem- 

 perature equilibrium curve. G. Tammann (An- 

 nalen der Physik, December, 1901) tests these 

 views by measurements of the plasticity of ice, 

 phosphorus, naphthalene, and piperine, and states 

 that the hypotheses above mentioned are untrue. 

 With ice and white phosphorus the plasticity in- 

 creases rapidly with the force and the tempera- 

 ture, while with the other substances there is a 

 maximum of plasticity as the pressure increases 

 at certain temperatures. 



Change of State. J. E. Mills (Journal of Phys- 

 ics and Chemistry, April) concludes that the total 

 energy per se of a molecule must be the same in 

 the liquid as in the gaseous state at the same tem- 

 perature, and he thus believes that in general 

 the energy required to change a liquid into gas is 

 spent in overcoming the external pressure and in 

 altering the distance apart of the molecules. As- 

 suming that the attraction between the molecules 

 varies inversely as the square of their distance 

 apart, he deduces a formula that affords a means 

 of testing the above assumption. The results are 

 favorable to it and also show that the absolute 

 molecular attraction is only slightly affected by 

 changes in temperature, as it depends primarily 

 on the chemical constitution of the molecule and 

 not upon its mass. 



Specific Heat. Ponsot (Comptes Rendus, March 

 24), from discussion of a gaseous homogeneous 

 mixture of bodies in chemical equilibrium, de- 

 duces the general law that at absolute zero two 

 systems of solid bodies comprising the same ele- 

 ments have the same specific heat. The specific 

 heat of a compound in the solid state is thus 

 equal to the sum of the specific heats of its sepa- 

 rate elements in that state. The specific heat of a 

 solid body and that of its saturated vapor both 

 tend toward zero for absolute zero of tempera- 

 ture. 



Light. Nature of White Light. M. Planck 

 (Annalen der Physik, February) considers that 

 the two apparently contradictory views of Gouy 

 and Carvallo on this subject may not be incom- 

 patible. The necessary feature is not the exist- 

 ence of interference of partials, but its regularity. 

 The author concludes that normal white light of 

 constant intensity is completely defined by the 

 energy distribution in the various parts of the 

 spectrum, and by the law that, within a small 

 part of the spectrum in which the energy distribu- 

 tion may be considered uniform, the energies and 

 phase constants of the single simple periodic par- 

 tials into which the light vector can be broken up 

 are absolutely irregular. 



Velocity. Cornu throws doubt upon the trust- 

 worthiness of results obtained by the revolving- 

 mirror method of measuring the velocity of light, 

 on the grounds (1) that the mirror may be ex- 

 pected to drag the beam with it, and thus make 

 the deduced velocity too small; (2) that the 

 beam incident on the fixed mirror moves across 

 it with a velocity comparable to that of light; 

 (3) that the aerial vortex around the revolving 

 mirror may modify the path of the rays. H. A. 

 Lorentz (Archives Neerlandaises, 6, 1901) takes up 



these criticisms one by one. As regards the first, 

 he shows, by geometrical optics, that the effect 

 of the rotation is to alter slightly at each in- 

 stant the distance between the center of the re- 

 volving mirror and the image formed by reflec- 

 tion, but not to alter the direction in which this 

 image lies. As regards (2), he calculates the 

 velocity with which the beam traverses the fixed 

 mirror, and maintains that, even if much greater, 

 it would not affect the directions of the reflected 

 rays. As regards (3), the author maintains that 

 as the vortex is symmetrically distributed 

 round the axis of revolution it will not tend 

 either to advance or retard the direction of either 

 the incident or reflected beam. A. A. Michelson 

 (Philosophical Magazine, March) discusses the 

 accepted values of the velocity of light, the ratio 

 of the two electrical units, and the velocity of 

 Hertzian waves, and concludes that new deter- 

 minations are necessary for accuracy. % A new 

 method free from the drawbacks of previous ones 

 would be virtually a combination of the Foucault 

 and Fizeau methods. Light passes from a slit 

 through a lightly silvered glass-plate and a lens 

 and falls on the upper half of a revolving mir- 

 ror. It is reflected to a grating which reflects 

 to the lower half of the mirror, and the beam 

 passes by way of a lens to the distant mirror. 

 The author hopes that the velocity may thus be 

 measured to, within about 5 kilometers. 



Absorption. G. E. Hale (Astrophysical Jour- 

 nal, April) believes that selective, like general ab- 

 .sorption, is a function of wave-length. In his 

 photographs of the spark spectrum of iron in water 

 the reversals first appear at the more refrangible 

 end of the spectrum, and as the conditions become 

 more favorable, lines of greater \vave-length are 

 reversed. Similar results were obtained with 

 other metals. This is interesting in view of 

 Campbell's discovery that in the spectra of cer- 

 tain stars the ultra-violet lines of hydrogen are 

 dark, while those of greater wave-length are 

 bright, which may be explained on the assump- 

 tion that the law of selective absorption resem- 

 bles that of general absorption. E. Hagen and 

 H. Rubens (German Physical Soeiety, March 7) 

 have examined the infra-red, visible and ultra- 

 violet rays from an arc lamp, before and after 

 interposition of chemically deposited metallic 

 films, by means of a special spectroscope. For 

 equal thicknesses gold is the most transpar- 

 ent to visible rays, platinum the most absorb- 

 ent. At its maximum transparency silver is 

 1,200 times as transparent as platinum. The 

 sequence of the metals as regards absorption i* 

 the same as for electric conductivity. R. S. 

 Clay (Royal Society, London. Nov. 8, 1901) 

 finds that in the production of color by suc- 

 cessive absorption, the loss of light proceeds 

 by geometrical progression, not by successive 

 subtraction. This, he says, is important ia 

 " process " three-color printing, where the inks 

 are always printed full strength, and the tint is 

 regulated by the size of the dots. " It is prac> 

 tically impossible to control the relative placing 

 of dots of different colors. This will be immr- 

 terial if the spectral absorption regions of the 

 three inks do not overlap, but if they do over- 

 lap, thon where dots of different colors coincide 

 there will be blackness, and the coloring wil 

 only be correct when the dots fall clear of one 

 another. Further reasons for using inks with 

 abrupt absorption are that the colors will be 

 purer, and that variation in the amount of ink 

 from impression to impression will not be so 

 important, thus avoiding a great practical diffi- 

 culty." To make the luminosities balance, Mr. 



