704 



PHYSICS IN 1899. 



hood of the surface of a body consisting of iso- 

 lated molecules the forces exerted by which on 

 an external molecule are functions of the dis- 

 tances the force function is periodic: hence there 

 will be a large number of stable positions for the 

 external molecule, separated by unstable ones. 

 If the body, therefore, move past the molecule, 

 the molecule's kinetic energy will be increased by 

 the same amount every time the body advances 

 through a molecular distance. The development 

 of heat by friction thus follows from molecular 

 theory. 



Elasticity. Weston (Physical Review, 5) has 

 determined the modulus of elasticity for copper, 

 brass, and steel under small loads by means of 

 interference methods, and concludes that for the 

 first two, and probably for the last, the ratio 

 between deflection and load is constant for loads 

 from 0.5 gramme to those that give the bar a 

 permanent set. 



Density. Berthelot (Comptes Rendus, 

 CXXVIII, p. 553) employs the density of liquids 

 to calculate their molecular weight, using an 

 equation connecting density with corresponding 

 pressure and absolute temperature, and with crit- 

 ical pressure, temperature, and volume. He can 

 obtain molecular weights within a few per cent. 



Torsion. Bouasse (Journal de Physique, Au- 

 gust) finds that the torsion produced by a given 

 couple depends on the mode of variation of the 

 torsion with respect to the time. If wires of the 

 same material but different diameters be loaded 

 with weights proportional to their cross sections, 

 and be given any torsions in the inverse ratio of 

 their radii (so that the radius of the wire multi- 

 plied by the angle of torsion is the same for all), 

 then this variation is the same at each instant 

 for all. Unless the time is taken into account, 

 experiments on torsion are of little utility. 

 Moreau (Comptes Rendus, CXXVIII, p. 292) 

 finds that the laws of torsion for steel are analo- 

 gous to those for soft iron, with the difference 

 that permanent torsion sets in later with steel. 



Ductility. Bouasse (Comptes Rendus. 

 CXXVIII, p. 291) finds that the limiting pull at 

 which permanent stretching occurs in the case of 

 copper wire decreases with prolonged heating. 



Liquids. Surface Tension. Pockels (Wiede- 

 mann's Annalen, LXVII, 3, p. 608) determines 

 the tension of a surface of separation between 

 two liquids by means of the cohesion balance, 

 either by dipping an adhesion ring into the lower 

 liquid and lifting it or by allowing one liquid 

 to expand on a contaminated water surface, re- 

 ducing the surface tension of the water till ex- 

 pansion ceases. The remaining surface tension 

 of the water is measured with the cohesion bal- 

 ance, and the tension between the two liquids 

 is obtained by deducting that of the upper liquid 

 from the surface tension of equilibrium. It is 

 found that the intermediate surface tension rap- 

 idly falls with the lapse of time. Grunmach 

 (German Physical Society) determines the sur- 

 face tension of liquids and of molten metals bv 

 observing interference waves due to two vibrat- 

 ing centers connected to the prongs of a tuning 

 fork. Some of his results are as follow: 



Fischer (Wiedemann's Annalen, July), in a paper 

 on the smallest thickness of liquid films, attempts 

 lo harmonize the results of various observers by 

 supposing that when a drop of oil is placed on 

 a surface of pure water a " precursory him " fir>t 

 spreads over the water, and is followed HHMV 

 slowly by another. In his own experiments he 

 used a pure mercury surface. 



Solution. Dieterici (Wiedemann's Annalen, 

 LXVII, 4, p. 859) concludes from experiments that 

 within the degrees of concentration observed 

 (normal to decinormal) there is in electrolytes no 

 decrease of vapor pressure with decrease of con- 

 centration, and that Raoult's law, according to 

 which the decrease of vapor pressure is independ- 

 ent of the nature of the solute, does not apply 

 to aqueous solutions of electrolytes. 



Crystallization. Poynting and Gray (Proceed- 

 ings of the Royal Society, LX1V, p. 121) have 

 sought for a directive action of one crystal on 

 another, rotating one quartz crystal near a sec- 

 ond one, and watching for forced oscillations in 

 the latter. The results were negative. 



Solidification. Wilson (Proceedings of the 

 Cambridge Philosophical Society, X, p. 25) finds 

 that the rate of solidification in a supercooled 

 liquid depends on the difference between the in- 

 ternal pressure in the solid and that in the liquid, 

 which difference in the case of water and ice at 

 1 C. amounts to 12 atmospheres. Phenomena 

 resembling explosion occur when, at very low 

 temperatures, solidification is retarded by vis- 

 cosity and the supercooling is gradually dimin- 

 ished. 



Oases. Kfflu.r. Embden (Wiedemann's Anna- 

 len, LXIX, 1, p. 204) has observed the phenomena 

 of periodic condensation in the efflux of gases 

 from narrow openings, photographing them by 

 Dvorak's method 01 striae. Stationary sound 

 waves are produced in all gases as soon as the 

 pressure exceeds 1.9 atmosphere. 



Viscosity. Breitenbach (Wiedemann's Anna- 

 len, LXVII, 4, D. 803), using the method of 

 transpiration, finds that the viscosity of ga- 

 proportional to a power of the absolute t<-ni|> ra 

 ture whose index in various gases lies bet \\een 

 0.0 and 1. The sphere of influence of a moleeule 

 therefore decreases as the temperature inen < 



Condensation. Mueller-Erzbach (Wiedemann's 

 Annalen, LXVII, 4, p. 899) finds that the alt ra- 

 tion of wax for vapor of carbon disulphide i- -till 

 felt though the wax be painted with a layer of 

 gum. The attraction lessens, however, with in- 

 creasing thickness of the layer, and when this 

 reaches 0.35 millimetre is no longer felt. The at- 

 traction was gauged by the ability of the gum- 

 covered wax to condense the vapor, which was 

 kept 1 to 2 colder than the wax. 



Barometry. Koch (Wiedemann's Annalen, 

 LXVII, 2, p. 485) has devised a standard ban. me 

 ter in which a faulty vacuum can be restored 

 by means of a permanently attached Sprcngrl 

 pump. 



Acoustics. Inten-ftitj/. The experiment- 

 Rayleigh (Philosophical Magazine, *XLVII. p. 

 308) negative Wilmer Duff's theory that ra.iia 

 tiorf is the cause of deviation from the law of 

 inverse squares in observations on sound in- 

 tensity. He attributes this deviation to atmos- 

 pheric refraction and reflection. 



Propagation. Vieille (Comptes Rendus, 

 CXXVIII, p. 1437). from a study of intense A\ 

 of compression produced by the' explosion of gun- 

 powder in a closed iron tube, concludes that tlio 

 pressure rises more and more rapidly in the wave 

 front as it moves away from the point of ex- 

 plosion. The velocity of the disturbance is also 



