PHYSICS, PROGRESS OF, IN 1890. 



711 



VISCOUS FLUIDS. 



Marine glue 2xlO >8 



SOLIDS. 



P;iraffine at '20 >2xlO u 



Hard steel, glass. 



etc WtoGxlO 17 



Soft steel. . 6 x 10 17 to x 10 18 



slight elongation, with large relative rotation of 

 the ends. Other complicated phenomena were 

 also observed. Ayrton and Perry find that a 

 double-twisted strip of constant length, with in- 

 itial pull, can be used as a delicate thermome- 

 ter or galvanometer, owing to its sensitiveness 

 to heat and to the electric current. 



Viscosity. Dr. Carl Barus, of the United 

 States Geological Survey (" Philosophical Maga- 

 zine," April), has made measurements of the ab- 

 solute viscosity of matter in all physical states 

 by new methods. He remarks that' viscosity is 

 the chief variable of our material environment, 

 and that it nowhere fails to appeal to the senses. 

 It furnishes the chief criterion of the physical 

 state of a body. His results are as follow, given 

 in C. G. S. units : 



GASES AND VAPORS. 



Ether at 6'8 x 10- 5 



Hydrogen at 0.. . 8'7 x 10- 



Air at tr 1-75 x IO-* 



Oxygen at 2-12 x 10~* 



LIQUIDS. 



Ether at 30 9xl()-< 



Ether at 10 1-9x1(1-3 



W;iterat97 3-0 x 10-3 



Water at 20 TO x 10 2 



The viscosity of gases thus lies between 7XlO~ 5 

 and 2X10- 4 , that of liquids between 9 X 10- 4 and 

 1 X 10-*, and that of solids between 10 and 

 10 18 . The viscosity of gases decreases on cool- 

 ing, whereas that of liquids increases. 



Liquids and Gases. Solution. Two opposite 

 theories of the solution of solids in liquids are 

 still held, and each is supported with ability. 

 The upholders of the chemical or "hydrate" 

 theory, who assert that the solid forms definite 

 compounds with the liquid, and that every so- 

 lution is a mixture of two or more of such hy- 

 drates, find new evidence in their support in the 

 curved figures that represent the properties of 

 solutions of various strengths. Such curves 

 show sudden changes of curvature at certain 

 points, which are the same whatever property 

 be examined, and so correspond, it is claimed, to 

 definite hydrates? On the other hand, the ad- 

 vocates of the physical theory who hold that 

 there is no chemical union, and that the mol- 

 ecules of the dissolved substance mingle freely 

 with those of the solvent in all proportions 

 claim that, at any rate in weak solutions, the 

 dissolved substance obeys laws analogous to those 

 of gases (see "Annual Cyclopaedia," 1889, page 

 692), and that therefore its molecules must be 

 uncombined with those of the solvent. But 

 even the most active opponents of the chemical 

 theory do not altogether deny the existence of 

 hydrates in strong solutions, and probably the 

 truth is to be found in a union of the two views. 

 Prof. S. U. Pickering (London Physical Society. 

 March 7), in an argument against the physical 

 theory, points out that while, if it be true, the 

 molecular depression of the freezing point of a 

 solution should be independent of the nature of 

 the solvent, the variations, in fact, are from 30 

 to 60 per cent. He says that on the theory he 

 opposes it should also be independent of the 

 amount of the solvent, but that there is a varia- 

 tion of 40 per cent. J. Stefan (Vienna " Berichte," 

 Nov. 21. 1889) has experimented on the velocity 

 of solution of a prism of rock salt that is inclosed 



in metal except at one end. When the free sur- 

 face was uppermost, the speed of solution was 

 inversely proportional to the dist;m<-<- <,i it,,. sur . 

 face from the open end; but when it pointed 

 downward, the velocity was uniform. A pri-in 

 a metre long would require seventy years for com- 

 plete solution in the formercase, but only two and 

 a half days in the latter ; the time varying in one 

 case as the square of the lineal magnitude, and in 

 the other in a simple rat io. Dr. Schultze reports in 

 " Science " his discovery that ordinary beer will 

 dissolve glass. After standing for a few minutes 

 m a glass vessel, the liquid changes perceptibly 

 in taste and odor. This is due to the lead in the 

 glass. One cubic centimetre of beer dissolved in 

 five minutes 6*26 ten millionths of a milligram me 

 of glass. In experiments on solutions of gases, P. 

 Van Buchem (Paris Academy of Sciences, Jan. 

 13), shows that there is a special state of equilib- 

 rium for such solutions when the lower part is 

 cooled and the upper is heated. 



Capillarity. Lord Rayleigh has made a series 

 of valuable experiments on the surface tension of 

 liquids. To test the hypothesis of Marangoni 

 (1871) that the great extensibility of a soap solu- 

 tion is due to the formation of a coating that 

 is caused by the chemical action of the carbon 

 dioxide in the atmosphere, he made observations 

 on a recently formed surface, thinking that the 

 formation of such a pellicle would require time. 

 The wave length of the stationary vibrations of 

 a jet of the liquid enabled him to calculate the 

 tension, and thus in this case the surface could 

 be observed before it was , & second old. Exact 

 measurement was rendered easy by photography, 

 and it was found that the tension of a surface so 

 recently formed was the same whether the liquid 

 were soap'solution or pure water. This evident- 

 ly supports Marangoni's hypothesis. The same 

 observer (Edinburgh Royal Society, March 27) 

 has observed the action of oil on the motion of 

 camphor on water. Cleanliness is absolutely es- 

 sential to this motion, which is caused by a' dif- 

 ference of tension between the pure wafer and 

 that which has dissolved the camphor. A mere 

 touch of the finger gives enough oil to the water 

 to stop the motion. The thickness of the film of 

 olive oil necessary to stop the motion was found 

 to be from 1-5 to 2 micro-millimetres. This 

 thickness is far below the range of the ordinary 

 forces of cohesion, for the black portions of a 

 soap bubble are 12 micro-millimetres thick. Yet 

 thinner films were experimented on later (Lon- 

 don Royal Society. June 5) in observations on 

 the superficial viscosity of water. Prof. Willard 

 Gibbs, of Yale, has claimed that this also is due 

 to a superficial pellicle, and Lord Rayleigh wished 

 to find whether such a pellicle was caused by 

 contamination. The viscosity was observed by 

 watching the rotation of disks or of rings of wire 

 just touching the surface of the liquid. Water 

 showed no resistance to a shearing stress as exert- 

 ed by a disk, but quickly stopped a ring with a 

 cross wire, which by rotation tended to deform 

 part of the surface. With a solution of saponine. 

 however, the surface acted as if almost rigid. 

 When the surface was cleaned by means of an 

 air blast, the superficial viscosity disappeared. 

 The films causing this viscosity were only & as 

 thick as those necessary to stop the motion of 

 camphor, measuring ^ micro-millimetre or 6 



