546 



PHYSICS, PROGRESS OF, IN 1902. 



be sharply determined by placing the bulb of a 

 sensitive mercury thermometer in the water just 

 as it emerges from the tube; when the motion be- 

 comes sinuous, even for an instant, the mercury 

 column shoots up. 



Surf ace- Tension. The surf ace-tensions of mixed 

 liquids have been investigated by W. H. What- 

 mough (Zeitschrift ftir physikalische Chemie, Dec. 

 5, 1901), who has made his measurements by de- 

 termining the pressure necessary to drive a stream 

 of air-bubbles through a capillary point immersed 

 in the liquid. He finds that under certain condi- 

 tions the method is capable of exceedingly great 

 accuracy. This experimenter was unable to con- 

 firm the results of Quincke and Harnack, who 

 found a difference in the surface : tension of fresh- 

 ly prepared solutions and those that have been 

 kept some time. The maximum deviation ob- 

 served by Whatmough after twenty-four hours in 

 six solutions was only 0.1 per cent. Mixtures of 

 sulfuric acid and water were found to exhibit a 

 remarkable maximum of surface-tension at 46 per 

 cent. H 2 SO, corresponding with a minimum of 

 compressibility. Minima occur in mixtures of 

 acetic acid with ethyl iodide, carbon tetrachloride, 

 benzene and chloroform, toluene with xylene, and 

 other mixtures of hydrocarbons. 



Viscosity. A. Batschinski (Moscow Imperial 

 Society of Natural History, Bulletins 1 and 2, 

 1901) has tested the law that the product of the 

 internal friction of a liquid and the third power 

 of the absolute temperature is a constant. He 

 finds that the relation holds with a large number 

 of substances, including bromine, nitrogen-diox- 

 ide, most halogen derivatives, and certain alde- 

 hydes and ethers; but anhydrides, acids, alcohols, 

 and water (below its boiling-point) in general do 

 not obey the law. P. Duhem (Comptes Rendus, 

 May 12) defines a fluid as a body, each element 

 of which is in a state completely defined by the 

 temperature and the density. Within an incom- 

 pressible fluid the virtual work of viscosity is zero, 

 and if a body is rigorously fluid and rigorously 

 incompressible, it must be considered devoid of vis- 

 cosity. All viscous fluids are compressible. The 

 author shows that the laws of motion of a viscous 

 fluid differ from those of a non-viscous only in 

 that there is no longer a relation in finite terms 

 between the pressure, temperature, and density, 

 such relation being replaced by a differential equa- 

 tion. For a compressible perfect fluid the density 

 at any point has the same value as if the fluid 

 were in equilibrium under the same pressure and 

 temperature; while for a viscous fluid in motion 

 the density at each point varies in such a sense as 

 to approach that value. In a slightly viscous 

 fluid, where the velocity varies little, these two 

 values are very nearly equal, and thus a perfect 

 fluid is the limiting form of a fluid slightly vis- 

 cous. The same author, in a later paper (ibid., 

 June 2), extends these considerations to a fluid 

 near the critical state. It is shown that while 

 the density differs by a finite amount from the 

 equilibrium value, the density of an element may 

 vary very slowly. The fluid is thus in quasi-equi- 

 librium, but such a state is not permanent. In a 

 fluid with sensible motion, if the accelerations are 

 large the rate of variation of density is no longer 

 very small, and a stirred fluid near its critical 

 state thus presents the same problem as that of a 

 dissolved substance diffusing slowly through the 

 solvent. The strife in a fluid near the critical 

 point are thus similar to the motions in a solution 

 having great differences of concentration at differ- 

 ent points. 



Gcldliiiizdlion. J. M. van Bemmelen (Archives 

 NSerlandaises, 6, 1901), has shown that silica jelly 



really consists of an open microscopic network, 

 or mass of polygonal cells, enclosing a large quan- 

 tity of water, which is not in chemical combina- 

 tion, and can be replaced by other liquids. When 

 slowly dried at ordinary temperatures, the jelly 

 at a certain point, called by the author the 

 " change-point," loses transparency because air en- 

 ters the cells and condenses there, but it becomes 

 clear again when this is displaced by a liquid. 

 The jelly may be dried till only one molecule of 

 water lo ten of silica remains, but exposure to 

 aqueous vapor causes the cell cavities to gradually 

 refill with water, the volume of the jelly remain- 

 ing almost unchanged. The phenomena can be 

 repeated indefinitely. The position of the change- 

 point depends on the structure of the jelly and 

 the constitution of the cell-walls. The power of 

 absorption of vapors is reduced by heat, and at 

 low redness may be entirely removed. S. Levites 

 (Russian Journal of Physics and Chemistry) stud- 

 ies the delay of gelatinization of some colloidal 

 bodies from an aqueous solution, produced by the 

 addition of salts or other substances. This effect 

 increases with the temperature, and the gelatini- 

 zation process may often be wholly prevented. 

 The author suggests that a colloid will undergo 

 gelatinization from a salt solution more slowly, 

 the more readily it dissolves in this solution, and 

 striking analogies between this process and crys- 

 tallization are pointed out. 



Crystallization. G. Tammann (Annalen der 

 Physik, April 29) believes that the so-called 

 " liquid crystals " are not such in reality. He 

 points out that the existence of crystals with a 

 displacement elasticity equal to zero would intro- 

 duce a fundamental modification of the space-net 

 theory, if it did not, indeed, require its complete 

 abandonment. Experiments, he asserts, support 

 the view that " liquid crystals " must really Li- 

 regarded as emulsions. A. Amerio (Nuovo C'i- 

 mento, November-December, 1901) reexamines 

 this question of " liquid crystals " by using an ap- 

 paratus similar to the ice calorimeter of Bunsen, 

 with which he demonstrates that at the points of 

 transformation from the clear to the turbid stato 

 there is an evolution of heat, and an absorption 

 on the reverse change. The specific heats of tin; 

 compounds examined were less in the former state. 

 Double refraction and dichroism are properties of 

 the turbid liquids and not due to solid particle-, 

 and the author believes that surface-tension H 

 the determining force of the orientation, and that 

 the droplets are not analogous to crystals. He 

 prefers to term them simply birefractive and 

 anisotropic liquids. P. R. Heyl (Physical Review, 

 February) has inquired experimentally whether 

 electrostatic stress might not alter the interfacial 

 angles and density of crystals formed under its 

 influence. Mercuric iodid, a salt sensitive to 

 slight mechanical disturbances, was employed as 

 indicator, but no effect was found. Hence any 

 molecular forces called into play in a solution un- 

 der electrostatic stress are not comparable with 

 the forces of crystalline attraction. 



Hydrostatic Pressure. W. Ramsay (Archiv< 

 Neerlandaises, 6, 1901) describes an attempt to 

 determine whether fine-grained particles, having 

 incessant pedetic motion in a liquid, exert hydro- 

 static pressure. His method was to determiiie 

 the density of a colloidal solution in water fir-it 

 by a hydrostatic method, and then by the pyk- 

 nometer. The latter is found to give a highor 

 value, and although the difference is so small 

 as to be comparable with the errors of obser- 

 vation, the author concludes that the particles 

 by their impacts on the sinker exert hydrostatic 

 pressure. 



