PHYSICS, PROGRESS OF, IN 1892. 



635 



stationary with respect to the mirror it appears 

 to be obeyed." The discrepancy is small, but 

 ini^ht be detected by very delicate observation. 

 In general the shape of the incident wave is not 

 pnvisely the same after reflection in a moving 

 medium. To a parallel beam the mirror acts as 

 if slightly tilted; to a conical one as if slightly 

 curved. Experiments hitherto made will not 

 allow of a definite conclusion. Most of them 

 are indecisive; a few show contradictory results. 



M<il, cults. C. V. Burton ("Philosophical 

 Magazine," February) elaborates the hypothesis 

 that at i >ms are strain figures in the ether. He 

 supposes that "a given portion of matter con- 

 sists not of any individual portion of ethereal or 

 other substance, but of modifications in the 

 structure, or energy, or other qualities of the 

 ether, and when matter moves it is merely these 

 modifications which are transferred from one 

 portion of the ether to another." From this 

 hypothesis he succeeds fairly well in deducing 

 most of the known properties of matter. G. 

 Jftger (" Beibiatter der Physik," xvi, p. 345) sup- 

 poses that molecules may be droplets, formed by 

 impact from larger masses, which ultimately 

 acquire a definite mean size for a given tempera- 

 ture. If the droplets are all of the same size, 

 the drops from which they are formed must be 

 at least twice as large. Since the division is 

 possible only when the via viva of impact is 

 greater than the work of division, it is possible, 

 by determining the increase of surface when a 

 sphere is split in two, to arrive at some idea of 

 the ultimate size of the droplets. The results 

 are in harmony with received ideas as to molec- 

 ular magnitudes. The calculated diameters, in 

 10 - centimetres, are: Water, 51; ether, 76; 

 alcohol, 52 ; methyl alcohol, 37 ; carbon disul- 

 phide. 73 ; chloroform, 80 ; acetone, 31. W. 

 Sutherland ("Philosophical Magazine," May), in 

 developing an elaborate kinetic theory of solids, 

 finds it necessary to assume that molecules 

 shrink with rising and expand with falling tem- 

 perature. This makes melting comprehensible, 

 it being hard to understand otherwise why so 

 small a rise in temperature at a particular point 

 should transform a solid into a liquid. Ladislas 

 Natanson (" Philosophical Magazine," July) 

 thinks that as a general principle atoms and 

 molecules tend unless they are subjected to 

 mutual or external forces to assume that kind 

 of ultimate arrangement the (pure) probability 

 of which is greatest. He states Newton's first 

 law thus : " Every time a point does not move in 

 a straight line with a constant velocity, we say 

 that a force is acting on it " ; and he gives his 

 principle in analogous form, thus : " Every time 

 molecules or atoms assume some steady state, 

 the probability of which is not the greatest, we 

 say that force'is acting on them." 



Standard Gauge for Inappreciable. Magni- 

 tudes. G. J. Stoney (Dublin Royal Society, 

 March 16) supposes that Angstrom's normal map 

 of the solar spectrum be extended in both direc- 

 tions, so that the numbers read from zero to 

 10,000, and that an ordinate of 10- metre be 

 erected on the 1,000 point. From the summit 

 to the further extremity an inclined plane 10 

 metres long with a grade of 1 in 10,000,000 

 would thus extend, the distance of which from 

 any ray in the scale would be its wave length. 



The ordinate at three fourths of a metre from the 

 tip would be the average distance to which 

 molecules of air dart; that at 1 centimetre from 

 the apez is the average interval between gas 

 molecules at the usual temperature and pressure 

 of the air; that at 1 millimetre from the tip is 

 the diameter of a molecule of gas. In this way 

 a step may be taken toward mentally appreciating 

 these infinitesimal magnitudes. 



Mechanics. Elasticity. J. O. Thompson 

 (" American Journal of Science," January) has 

 measured the elastic lengthening of wires of 

 brass, silver, and copper 3 metres long by direct 

 observation with two fixed cathetometers. He 

 finds Hooke's law only approximately true, the 

 lengthening (a;) being nearly expressed by the 

 equation x = a P + j8 P* + y P 1 , where P is the 

 stretching weight. The moduli of elasticity thus 

 calculated may be 10 per cent, larger than the 

 ones generally received, necessitating a recalcu- 

 lation of many physical constants. 



Liquids. Solution. W. Ostwald, perhaps 

 the chief apostle of the physical theory of solu- 

 tion ("Annual Cyclopa?dia," 1891, p. *726), has 

 published his views in a volume. In reply to a 

 criticism by J. W. Rodger (Nature," Dec. 31. 

 1891), Prof. Ostwald says that he " can not at all 

 admit the existence of a contrast between the 

 two theories," and that the question as to whether 

 solution is a physical or a chemical process is 

 " unclear, and therefore very harmful." In a 

 subsequent letter (ibid., Feb. 16, 1892) he asserts 

 again that the whole matter may be treated en- 

 tirely independently of the question of possible 

 chemical interaction between dissolved substance 

 and solvent. Messrs. Wanklyn. Johnstone, and 

 Cooper (" Philosophical Magazine," November, 

 1891) examine a function which they name the 

 condensate. This equals i ?'i, where i is the in- 

 crement of weight due to the entrance of one 

 gramme of substance into 100 cubic centimetres 



of solvent, and t'i = 1 ^ rr-. They 



specific gravity 



show that the condensate bears always an atomic 

 relation to the gramme of the salt in question, 

 while in sugar, for instance, it is zero, since 

 i = t'i. They argue that in this last case, and in 

 all similar cases, there is true physical solution ; 

 otherwise not. Georges Charpy (Paris Academy 

 of Sciences, March 7) concludes that the varia- 

 tion of the density of a solution as a function of 

 the concentration is a complex phenomenon and 

 can not be used to study the state of the body, 

 and that there* is no reason why a solution of 

 maximum density should be regarded as a 

 hydrate. S. U. Pickering (" Philosophical Maga- 

 zine." July) shows that the solution of gas always 

 evolves heat, and concludes that it is a chemical 

 phenomenon. He thinks that the only bar to 

 the complete acknowledgment of the chemical 

 theory is the question of how chemical union 

 could produce such a state of the dissolved 

 matter as is shown to exist by the facts brought 

 up by adherents of the physical or osmotic 

 theory. He considers, however, that he has 

 already overcome this difficulty in a communi- 

 cation to the German Chemical Society, main- 

 taining that this very quasi-independence of the 

 dissolved substance "is a direct consequence of 

 the formation of highly complex nydrates. 

 Picton ("Journal of the Chemical Society," 



