

PHYSICS, PROGRESS OF, IN 1902. 



545 



of more than 100 milligrams, and will serve to 

 weigh to one-thousandth of a milligram. The 

 balance has an arrangement that maintains the 

 flexure after unloading. The loss of weight of 

 musk by volatilization is clearly demonstrated 

 by this instrument, which shows it to be propor- 

 tional to the time. 



Density. G. Guglielmo (Atti del Lincei, Dec. 

 1, 1901) describes a balance used under water to 

 determine densities. As the weight in water can 

 be made small, the friction on the knife-edges is 

 small, and these are, moreover, lubricated by the 

 water. The balance is thus of extraordinary deli- 

 cacy, and the error due to surface tension on the 

 suspending wire is also avoided. The balance 

 arm is of glass, with a central bulb to give the 

 whole suitable weight. The knife-edges of the or- 

 dinary balance are replaced by points resting on 

 planes. 



Strain. L. Cailletet (Comptes Rendus, Feb. 

 17) notes that a thick coating of strong glue 

 spread upon glass splits off when it dries, pulling 

 with it thin layers of the glass, and leaving a 

 decorative curved pattern, especially if crystalline 

 salts are added to the glue. A cylindrical vase 

 of thin glass so treated will split into a hemicyl- 

 inder, and a thick piece of glass will, under polar- 

 ized light, show the contractile strain to which 

 it is subjected. 



Torsion. Coker (Edinburgh RoyaL Society, 

 Nov. 12, 1901) has studied turned steel bars about 

 one-half inch in diameter, under torsion, together 

 with tension and bending. Such experiment has 

 generally been limited hitherto to wires. According 

 to the usual theory, if the material changes from 

 the elastic to the plastic condition at the yield- 

 point, the maximum torque which the bar will 

 stand, when it is all plastic and subject to the same 

 shearing stress, is four-thirds the value at which 

 the first marked deviation from perfect elasticity 

 occurs. With an iron bar, the deviation occurred 

 at 375 inch-pounds and failure at 525 inch- 

 pounds, the ratio being 1.4; a steel specimen gave 

 675 and 870, a ratio of 1.29. The phenomenon of 

 breakdown and subsequent recovery from over- 

 strain is similar in torsion to that in tension, 

 such a moderate heating as 100 C. having a 

 marked effect in promoting the recovery. The 

 experiments demonstrate that the limits of elas- 

 ticity do not remain in their original positions; 

 stress carried beyond the elastic limit in one direc- 

 tion reduces the other limit to zero. The theory 

 that a twisted bar is twice as strong to resist tor- 

 sion \n the same direction as in the opposite one 

 is not borne out by experiment. As for the effect 

 of tension on torsion, a tension within the elastic 

 limit has practically no effect on the elastic part 

 of the torsion curve, but lowers the yield-point. 

 The effect of torsion on tension was only exam- 

 ined below the elastic limit. No difference was 

 observable, whether the bar were twisted or not, 

 provided its elasticity remained unimpaired. 

 When a bar was bent within the elastic limit, and 

 tested by twisting beyond that limit, its yield- 

 point was considerably lowered, and the effect was 

 to give the bar a permanent set in bending under 

 the same moment to which it had previously been 

 elastic. 



Cyclonic Motion. J. Aitken (ibid., 40, 1, 1901) 

 notes that vortices will not occur either in air or 

 water unless there is initial circular motion as 

 well as low pressure. Otherwise the fluid will 

 flow toward the low-pressure center radially. 

 This may be illustrated by emptying a basin of 

 water from an orifice at its lowest point. In 

 water vortices so formed there is a great increase 

 in velocity near the center. The resistance of- 

 VOL. XLIL 35 A 



fered in the case of a cyclone by the spirally 

 moving air enables the latter to develop more 

 energy than if the air moved inward radially, 

 as the retardation causes a fall of pressure, and 

 the energy of the cyclone is increased. 



Liquids. Solution. Friedliinder (Zeitschrift 

 fur physikalische Chemie, Oct. 1, 1901) has ob- 

 served certain peculiarities of partially miscible 

 liquids near the critical point, using chiefly mix- 

 tures of water and isobutyric acid. The tempera- 

 ture coefficient of internal friction increases very 

 greatly in the neighborhood of the critical point, 

 not only with these substances, but also with 

 mixtures of phenol and water, and with benzene, 

 water, and acetic acid. Precisely similar results 

 were obtained on studying the opalescence or milk- 

 iness that appears when such mixed liquids are 

 cooled near the critical point. In mixtures of 

 isobutyric acid and water the opalescence reached 

 a maximum of 69 units for a critical mixture 

 0.04 C. above the critical temperature, and fall- 

 ing to 4.5 units at a temperature 1 above. The 

 density, coefficient of expansion, electrical con- 

 ductivity, and refractive index of the solutions, 

 however, change continuously in the neighborhood 

 of the critical point, and do not exhibit maxima 

 or any other abnormal features. 



Wave Motion. W. G. Fraser (Philosophical 

 Magazine, October, 1901) has sought to explain 

 the fact that large waves instead of being re- 

 flected at an obstacle, as a small wave is, and as 

 all waves should be, according to theory, break 

 into spray. His explanation is that the vertical 

 component of velocity is checked by the obstacle, 

 which tends to produce variation of density, and 

 finally breach of continuity. For deep waves 

 with direct incidence it is found that cohesion will 

 prevent rupture so long as the ratio of the ampli- 

 tude to the wave-length is small. In the case of 

 oblique incidence not only the vertical component 

 of velocity, but also the horizontal component par- 

 allel to the obstacle is checked, and this somewhat 

 lessens the liability to break. 



Composition. G. F. Stradling (Franklin In- 

 stitute Journal, October, 1901) elaborates Ront- 

 gen's theory that water contains two kinds of 

 molecules, which he designates " ice molecules " 

 and " molecules of the second kind." This has 

 already been worked out quantitatively by Suth- 

 erland, and promises to explain- many points in 

 which water behaves oddly, such as the well- 

 known maximum density at 4 C. ; the minimum 

 of compressibility at 63 C. : the decrease by in- 

 creasing pressure of the coefficient of thermal ex- 

 pansion; the lowering by increased pressure, and 

 by dissolved salts, of the temperature of maximum 

 density ; the influence of pressure and temperature 

 on its viscosity ; the decrease of volume in prepar- 

 ing aqueous solutions; and the low specific heat 

 of solutions as compared with water. The au- 

 thor says : " Not only in the matter of solutions, 

 but in other more strictly physical relations, it is 

 a misfortune that the r6le of the typical liquid 

 was assigned to water." 



Flow. H. T. Barnes and E. G. Coker (Physical 

 Review, 12, 1901) have continued the investiga- 

 tions of the former author on the flow of liquids. 

 Barnes showed that if water be heated while flow- 

 ing through tubes in parallel stream-lines, the dis- 

 tribution of heat is not uniform; where the heat 

 is applied to the outside only a few layers of the 

 water will be heated, and where the heat is from 

 a central wire the hot water flows only along the 

 wire. Beyond the critical velocity, where the 

 flow is eddying and sinuous, the distribution of 

 heat throughout the water-column is uniform. 

 The authors now find that the critical velocity can 



