May 2 1, 1896] 



NATURE 



55 



information. Black Urjanis usually breed true ; when they fail to 

 do so, the progeny is generally a sport of a particular kind called 

 idjji-, uniforn-.ly red when young, more or less mottled with 

 white when adult. These dijjis are apt to " throw back," and 

 in turn produce good Urjanis. 



One of my own mottled birds (dijji) remains in my posses- 

 sion, and is now^ mated to an Urjani—xa. own brother. The 

 )iair has produced this spring four young : three dijjis, just like 

 the mother, and one partial reversion to the Uijaiii form, being 

 dark-chequered blue, with red bars on the wings. The original 

 parent pair of Urjanis have also raised four squabs this season 

 —three normal and one dijji ; sexes not yet determined. I 

 state these facts without comment ; but would be glad to know 

 whether fanciers in England or elsewhere have observed any- 

 thing quite as striking in the way of colour-variation. 



lieyrout, Syria. W. T. Van Dyck. 



Dependence of the Colour of Solutions on the Nature 

 of the Solvent. 



It is a well-known fact that the colour exhibited by one and 

 the same body in solution depends more or less on the nature of 

 the solvent. In some cases this phenomenon can be satis- 

 factorily accounted for by electrolytic dissociation, but in the 

 majority of cases hitherto examined this explanation is not 

 admissible. Perhaps the most striking of these is that of iodine, 

 the solutions of which are coloured variously violet, blue, brown, 

 and yellow. The hypothesis has been put forward that the 

 variation in absorption might be due to the formation of 

 molecular aggregates of variable complexity : but this, at least 

 in the case of iodine, has been rendered verj- improbable by the 

 recent researches of Beckmann and others. Nor does the 

 liypothesis that the variation may be due to a varying degree of 

 combination with the solvent seem much more promising. 



If, now, absorption be a case of electrical resonance, should 

 one not expect a relation between the absorption of the dissolved 

 body and the physical properties of the solvent, sufficient to 

 account for the observed variations? That such a relation 

 should exist, seems possible from the following rough 

 considerations, 



The period of vibration of an electric oscillator is, in the 

 usual notation, 



T = 2irv'LC, 

 where L = self-induction, and C = capacity. But now : — 



LC = ^Km, 

 where ^ is a geometrical factor and K and jx are the dielectric 

 constant and permeability of the surrounding medium. Also 

 li- = Kfi, where n is the index of refraction of the medium for 

 very long waves, whence it follows that 

 T = 2TrnsJg, 

 which means that the principal absorption-band should travel 

 towards the red end of the spectrum as the index of refraction 

 of the solvent increases. This result is identical with the 

 general qualitative law enunciated many years ago by Kundt, 

 on the basis of experimental data. There are, it is true, various 

 breaks in the parallelism ; still this mode of viewing the question 

 seems to offer more possibilities than the others. 



Holywood, Belfast. F. G. Donnan. 



Hatching Lizards' Eggs. 



Cax any of your readers suggest a way to hatch lizards' eggs? 

 I have had a pair of bright-green lizards (I think they came from 

 Italy) in a glass vivarium in a very sunny window for two years 

 and a'half. Last year, on May 19, the female laid eleven eggs. 

 I left them exactly as the mother laid them, and after about 

 three weeks I opened one and found the rudiments of a young 

 lizard ; but the other eggs never came to anything. I should like 

 to rear them this year if it is possible. H. A. Ross. 



Trevean, Penzance. 



THE DIFFUSION OF METALS. 



IT IS now quite usual to think of alloys as being solid 

 solutions and to recognise that the atoms of solid 

 metals are in active movement. That this must be the 

 case, is revealed by the passage of metals to allotropic 



NO. 1386, VOL. 54] 



modifications in which the physical properties differ 

 widely from those of the same metals in their normal state. 

 It is well, therefore, that we should remember how much 

 was done for us thirty years ago by Matthiessen in 

 framing such views, and by Graham in showing that 

 solid metals are true solvents for gases which move and 

 diffuse freely in them, sometimes to reappear with 

 gaseous elasticity. 



The experimental portion of the latter work, Graham 

 entrusted to me, and my hope that I should be able to 

 extend his work on the diffusion of salts, to liquid and 

 solid metals, has been somewhat tardily realised by the 

 delivery in the present year of the " Bakerian Lecture" 

 of the Royal Society, of which the following is a brief 

 abstract. 



Part \. — Di^iision of Molten Metals. 



In the first part of it allusion is made to some earlier 

 experiments of my own conducted in 1883 on the 

 diffusion of gold, silver, and platinum in molten lead. It 

 is'Strange that although the action of osmotic pressure 

 in lowering the freezing point of metals has been care- 

 fully examined, very little attention has been devoted 

 to the measurement, or even to the consideration, 

 of the molecular movements which enable two or more 

 metals to form a truly homogeneous fluid mass. The 

 absence of direct experiments on the diffusion of molten 

 metals is probably explained by the want of a sufficiently 

 accurate method. Ostwald has stated, moreover, with 

 reference to the diffusion of salts, that " to make accurate 

 experiments in diffusion is one of the most difficult pro- 

 blems in practical physics," and the difficulties are 

 obviously increased when molten metals diffusing into 

 each other take the place of salts diffusing into water. 



The continuation of the research was mainly due to 

 the interest Lord Kelvin had always taken in the experi- 

 ments. The want of a ready method for the measure- 

 ment of comparatively high temperatures, which led to 

 the abandonment of the earlier work, was overcome when 

 the recording pyrometer was devised, and the use of 

 thermo-junctions in connection with this instrument 

 rendered it possible to measure and record the tempera- 

 ture at which diffusion occurred. Thermo-junctions 

 were placed in three or more positions in either a bath 

 of fluid metal or an oven carefully kept hotter at the top 

 than at the bottom. In the bath or oven, tubes filled 

 with lead were placed, and in this lead, gold, or a rich 

 alloy of gold, or of the metal under examination, was 

 allowed to diffuse upwards against gravity. The amount 

 of metal diffusing in a given time was ascertained by 

 allowing the lead in the tubes to solidify ; the solid 

 metal was then cut into sections, and the amount of 

 metal in the respective sections determined by analysis. 



The movement in linear diffusion is expressed, in 

 accordance with Pick's law, by the differential equation 



t'L = /. ^' 

 dt dx" 



In this equation .r represents distance in the direction 

 in which diffusion takes place, ?' is the degree of concen- 

 tration of the diffusing metal, and / is the time ; /' is the 

 diffusion constant, that is, the number which expresses 

 the quantity of the metal in grams diffusing through unit 

 area (i sq. cm.) in unit time (one day) when unit differ- 

 ence of concentration (in grams per c.c.) is maintained 

 between the two sides of a layer i cm. thick. The ex- 

 periments described in the Bakerian Lecture showed that 

 metals diftuse in one another just as salts do in water, 

 and the results were ultimately calculated by the aid of 

 tables prepared by Stefan for the calculation of Graham's 

 experiments on the diffusion of salts, special tables being 

 calculated by one of my students, Mr. A. -Stansfield, in 

 connection with this research. 



The necessary precautions to be observed and the 

 corrections to be made were described at length and the 



