PHYSICS. 237 



parts, proportionally increasing the total surface. Supposing the mole- 

 cules to be spherical, he gets for their radius 0.000000005"°^, or 5 to G 

 (million)^ for the number in one cubic centimeter ; agreeing with the 

 results of Thomson and Maxwell. — {Am. J. Sci., Ill, xviii, 135, August, 

 1879.) In a subsequent paper, the same author gives a calculation of 

 the mean free path of the molecules in the vapor of water lying close to 

 the liquid surface, which he finds to be 0.0000024™". — {Am. J. ScL, III, 

 xix, 222, March, 1880.) 



MECHANICS. 



1. Of Solids. 



The motions of finely divided solid particles when immersed in liquids, 

 commonly called the Brownian movement, and those of dust-particles 

 in air, have both been attributed to the same cause, the shock of colliding 

 molecules. Niigeli has calculated from the data of the kinetic theory 

 what the velocity of the smallest fungus-particle observable with the 

 microscope should be, supposing an oxygen or nitrogen particle to come 

 in collision with it. Since such a particle is 300 million times heavier 

 than the molecule, the velocity would not be greater than that of the 

 hour hand of a watch. For ordinary motes, the motion would be 50 mil- 

 lion times slower. The same results essentially were obtained for liquids ; 

 whence the author thinks the Brownian movements are phenomena con- 

 nected with the surface tension of the liquid. — {Ber. Al: Milnchen, 1879, 

 389.) 



Buchanan has communicated to the Eoyal Society of Edinburgh a pa- 

 per on the compressibility of glass, using a rod of this material placed 

 in a receiver of steel with glass ends connected with a hydraulic pump. 

 The compression is read by means of micrometer microscopes, and the 

 pressure is determined by a manometer consisting of a mercurial ther- 

 mometer with a stout bulb, previously calibrated in a piezometer, the 

 bulb being immersed in the water under pressure, while the stem pro- 

 jects outside. The glass used was a flint glass, and the compression 

 was observed up to 40 atmospheres. The cubical compression observed 

 was, for each atmosphere, 2.92 parts in a million. Incidentally the 

 compressibility of water at 2^.5 C. was found to be 0.05160 volume per 

 thousand per atmosphere ; and its diminution with rise of temperature 

 was confirmed, being 0.04831 at 12o.5C.—(iV^a^Mre,xxii, 377, 1880.) Barus 

 has studied the influence of hardness upon the thermo-electric proper- 

 ties and the specific resistance of steel. The steel used was in the form 

 of wire and was hardened by heating it by an electric current, in a glass 

 tube through which a stream of carbon dioxide gas was passed and 

 then suddenly admitting water. The resistance and the electromotive 

 force vary with the hardness, so that if the electromotive force be taken 

 as a measure of the hardness and the magnetic constant of the steel, 

 the resistance is a linear function of it.— {Phil Mag., Y, viii, 341, No- 

 vember, 1879.) Marangoni, with reference to Bottomley's experunent 



