PHYSICS, PROGRESS OF, IN 1892. 



traction of gravity, and that in smaller bodies 

 the repulsion exceeds, so that two particles of 

 dust with radii of -j-g^g- millimetre repel each other 

 at 0" with a force a million times greater than 

 their attraction. This fact evidently must not 

 be neglected in treating of molecular forces, 

 complex though they may be. Sydney Young 

 (" Philosophical Magazine," February) finds as 

 the result of four years' work on the vapor pres- 

 sure of various benzenes and alcohols that Van 

 der Waals's generalizations concerning vapor 

 pressure do not hold strictly true. Van der 

 Waals showed, as he supposed, that if the absolute 

 temperatures of various substances be propor- 

 tional to their absolute critical temperatures, 

 their vapor pressures will be proportional to 

 their critical pressures, and their volumes (both 

 as liquid and as saturated vapor) to their critical 

 volumes. This was found nearly true for chloro-, 

 bromo-, and iodo-benzene when compared with 

 fluorbenzene ; only approximately for benzene, 

 carbon tetrachloride, stannic chloride, and ether ; 

 and not true at all for the alcohols and acetic 

 acid. Sir William Thomson (" Philosophical 

 Magazine," March) states that if the motion of 

 every particle of matter in the universe could 

 once be exactly reversed, the course of Nature 

 would thenceforward be reversed forever. The 

 foam bubble about to break at the foot of a wa- 

 terfall would reunite and glide upward with the 

 water to the top ; bowlders would be again trans- 

 ported to the ledge whence they were torn, and 

 be again resolved into the mud from which they 

 were formed. All these astounding results would 

 proceed from simple thermodynamic principles. 

 In the same article Sir William says that though 

 it is very improbable that in the course of a 

 thousand years one half of a bar of iron will of 

 itself become warmer by a degree than the other, 

 this certainly would happen in a very long time 

 if the bar were covered with a varnish impene- 

 trable by heat; but if it be not so covered, 

 and the number of molecules in the universe can 

 be considered as infinite, then this would never 

 happen. In a subsequent cirticle (ibid., May) he 

 shows by a test case the failure of the so-called 

 law that in the ultimate state of a system the 

 average kinetic energy of two given portions 

 must be in the ratio of the number of degrees of 

 freedom of those portions respectively. This 

 would be true only for a perfect gas, in which 

 the molecules move for comparatively long times 

 in apparently straight lines. 



Radiation. Angstrom has studied the heat 

 radiation of rarefied gases under the electric dis- 

 charge by means of the bolometer. With a given 

 pressure the radiation varies as the intensity of 

 the current. With a constant current, the radia- 

 tion does not vary while the pressure increases 

 from O'l to T5 millimetre, but at higher pres- 

 sures it increases somewhat. The ratio of the 

 intensity of the radiation of shorter wave length 

 to the whole decreases with increase of pressure. 

 J. T. Bottomley (London Royal Society) finds 

 that the loss of heat by convection and radiation 

 from a surface of soot is 3-42 X 10~ 4 C. G. S. 

 units per square centimetre, per second, per 1 C. 

 difference of temperature, with a difference of 

 100 and the surface at 14. In a vacuum of one 

 half millionth atmosphere, it was only 1'40 X 10~ 4 . 

 C. C. Hutchins (" American Journal of Science," 



May) has shown that the radiation of air increases 

 with the humidity and with accidental impuri- 

 ties, as carbon dioxide and gases from the de- 

 composition of dust. The absolute radiation was 

 2,562 X 10- 9 for a humidity of 90, and only 1,513 

 for a humidity of 47. The waves from heated 

 air must be very long, for he found that they 

 can not penetrate quartz. Ayrton and Kilgour 

 (London Royal Society, Nov. 19) have measured 

 the emissivity of thin 'wires in air, each wire be- 

 ing stretched along the axis of a horizontal 

 water-jacketed cylinder 32*5 centimetres long, 

 whose inner surface was black and kept at a con- 

 stant temperature. They arrived at the follow- 

 ing results: 1. Given the temperature, the emis- 

 sivity is higher the finer the wire. 2. For each 

 wire the emissivity increases with the tempera- 

 ture, the rate increasing with the fineness of the 

 wire. 3. Hence, the effect of the surface on the 

 total loss of heat per second, per square centi- 

 metre, per 1 C. excess of temperature, increases 

 as the temperature rises. 



Absorption. Some discussion has arisen as to 

 the efficacy of a solution of alum as an absorber of 

 heat rays. Its power has been taken for granted 

 in numberless experiments, but C. C. Hutchins, 

 of Bowdoin College, finds that it is only a slightly 

 better absorber than pure water, and that even 

 sheets of pure alum are little better. Raoul 

 Pictet (" Comptes Rendus," May 30) gives addi- 

 tional proof of his recent discovery that heat 

 waves of low temperature meet with little resist- 

 ance (" Annual Cyclopedia," 1891, p. 728). A 

 tube of chloroform is placed in a refrigerator 

 at 120 C. with a thermometer which indicates 

 only 68*5, while crystals form in the chloro- 

 form. But in a refrigerator at 80 the ther- 

 mometer falls to 80, and the crystals disap- 

 pear. The reason is that the latent heat of 

 crystallization prevents the thermometer from 

 falling in the first case. The thermometers used 

 employed alcohol and sulphuric ether, and were 

 checked by hydrogen thermometers. 



Conductivity. Alphonse Berget (Paris Acade- 

 my of Sciences, June 7) has measured the con- 

 ductivity of bars of rare metals by observing the 

 Newton's rings formed by the pressure of the end 

 against glass disks as they elongate. C. Barus 

 (" American Journal of Science," July) has meas- 

 ured the change of conductivity in thymol on 

 passing from the solid to the liquid state. The 

 substance being placed between two horizontal 

 copper plates, the lower one was suddenly cooled, 

 and the time rate at which the heat traveled 

 across was measured by a thermo-couple. His 

 results were as follow : 



k = absolute heat conductivity in . 

 en 



Solid (12) . . 10 x * = 859 



Liquid (13) 10 6 x k = 313 



c 2 

 x = thermometric conductivity in . 



Solid (12).... .. 10 x a: = l,077 



Liquid (13) 1U x = 691 



Increment on passing from liquid to solid 



Respectively -13 and -36 with reference to solid. 

 ' -15 and -56 " " " liquid. 



Regulation. H. Le. Chatelier (Paris Academy 

 of Sciences, Jan. 11) has developed formula? from 

 which it appears that compressed pulverized ice 

 in contact with a liquid or vapor that is less com- 



