July 15, 1922] 



NA JURE 



77 



custom, to be that of its intrusion. We are entitled, 

 however, to consider the previous history of the 

 material composing these intrusive Archaean masses, 

 and, in view of their predominantly banded structure, 

 which marks them off as in some way different from 

 later intrusive masses of similar composition, such 

 consideration seems forced upon us. 



Now the banded character of the Archaean gneiss 

 suggests a partial derivation by melting from some 

 stratiform materials such as sedimentary or volcanic 

 rocks, or at any rate from rocks showing marked 

 small-scale differentiation into basic and acid types. 

 I do not think that stratiform differentiation during 

 or previous to crystallisation can be seriously put 

 forward as a cause of the banding, in view of the 

 rarity of this phenomenon in more recent granites, and 

 the fact that in them it is largely a marginal effect. 



May we not then have in these Archaean gneisses 

 the recrystallised remnants of still older sediments 

 and lavas, and who is to say that they may not also 

 embrace portions of the original surface on which 

 water first settled, but so obscured by recrystallisa- 

 tion that the question of its molten or planetesimal 

 origin is now unsolvable ? 



The difference between the two views is simply 

 that one regards the history of sedimentation on the 

 earth as cut off sharply by intrusion, while the other 

 sees it extending still further back into the mists of 

 the past, beyond the point where human vision is 

 any longer capable of discrimination. Where, on 

 either view, is the decisive criterion between the 

 nebular and planetesimal hypotheses ? 



W. B. Wright. 



Manchester, June 27, 1922. 



Wegener's Displacement Theory. 



Wegener's speculations have attracted so much 

 attention that there must be many who would be 

 glad to find some simple means of testing his fittings 

 and coincidences for themselves. Owing to the 

 distortion present in all maps such tests must be 

 carried out on a globe. Wegener himself uses 

 tracing paper, which must be cut and slashed in 

 order that it may even approximately fit the surface ; 

 and any one who has tried it will admit that it is 

 difficult to obtain satisfactory results. An easier plan 

 is to roll out a lump of modelling wax or plasticine 

 into a sheet of moderate thickness. The sheet may 

 then be pressed upon the globe and cut to the required 

 shape. According to my own experience, the best 

 method is to cut the sheet a little smaller than the 

 area that is to be represented, so that the actual 

 margin appears all round it, and to build it outwards 

 to this margin by the addition of small pieces of wax. 

 Old plasticine which has become rather dry works 

 very well and does not stick to the globe. 



But much more precise tests can be carried out with 

 the help of some form of triangular compasses. The 

 three points of the compasses may be placed on three 

 critical points of the globe and afterwards transferred, 

 without altering their relative positions, to any other 

 part of the globe that may be desired. The ordinary 

 triangular compasses of the draughtsman are very 

 little use upon a spherical surface, but a fairly 

 convenient instrument can be constructed with an 

 ordinary one-jointed two-foot rule as its basis. 

 A point about an inch long is fixed near the joint, and 

 each arm is provided with a sliding carrier. Each 

 carrier bears a short sleeve through which a pointed 

 rod, such as a knitting needle, slides rather stiffly. 

 These rods form the other two points, and all three 

 should stand approximately at right angles to the 

 plane of the rule. 



NO. 2750, VOL. I 10] 



This is an easily constructed type, but much more 

 convenient forms can be devised. If, for example, 

 the arms are arcs of circles, of suitable diameter, so 

 that they may stand concentric with the globe, 

 the- points may all be of fixed length, and the most 

 troublesome of the adjustments required by the 

 straight-armed form will be avoided. 



This is not the place to discuss Wegener's views, 

 but the use of triangular compasses seems to show 

 that a rather high degree of plasticity is necessary 

 in the masses of " Sial " in order to produce the 

 coincidences on which he bases his calculation of the 

 probability that his theory is correct. 



Philip Lake. 



Sedgwick Museum, 

 Cambridge, June 21. 



Opalescence Phenomena in Liquid Mixtures. 



It is well known that liquids which mix completely 

 above a certain critical temperature, e.g. phenol and 

 water, exhibit a strong and characteristic opal- 

 escence as the temperature of the mixture is lowered 

 to a point slightly above that at which the com- 

 ponents separate. A quantitative theory of this 

 phenomenon was put forward by Einstein (Annalen 

 der Physik, vol. 33, 1910) on the basis of thermo- 

 dvnamical reasoning, the spontaneous local fluctua- 

 tions of concentration of the mixture being taken 

 into account and the light-scattering due to the 

 resulting fluctuations of refractive index being 

 evaluated. He obtained as the expression for the 

 light-scattering 



*- 2 (M/N\ 4 ) . ^fey/ ^gjf ^ ~ P er unit volume, 



where /j. is the refractive index of the mixture and 

 c (log P)/ck expresses the rate of change of the vapour 

 pressure of one of the components with concentration, 

 a quantity which becomes very large as the critical 

 temperature and concentration are approached, thus 

 giving rise to a marked opalescence. It should be 

 pointed out, however, that Einstein's expression does 

 not include the whole effect, for we have also to 

 consider the result of the fluctuation of density of either 

 component taken separately, and to add to Einstein's 

 formula 



(^/iSMRT/N^Q^V-iHV + s) 2 V1/ , % „ 



+/j 2 (m 2 2 -i) 2 (^ 2 2 +2) s ], 



where p lt /3 2 , %, m 2 are respectively the compressi- 

 bilities and refractive indices of the components. 

 Further, the light-scattering due to the anisotropy 

 and arbitrary orientation of the molecules of the 

 components lias also to be added. 



The result of these corrections of Einstein's 

 investigation may briefly be indicated. Very near 

 the temperature at which the mixture separates 

 into two phases, the fluctuations of concentration 

 contribute by far the larger portion of the effect. 

 But at higher and lower temperatures the effects 

 of fluctuations of density and molecular anisotropy 

 are no longer negligible, and when the temperature 

 is sufficiently removed from the critical point they 

 form a substantial part of the whole. Further, the 

 increase in relative importance of the effect of 

 molecular anisotropy in these circumstances should 

 result in an increase in the proportion of unpolarised 

 light in the transversely-scattered beam as we recede 

 from the critical temperature. 



The foregoing indications of theory have been 

 confirmed generally in a series of experiments over 

 a wide range of temperatures on light-scattering 

 in phenol -water mixtures undertaken under the 

 writer's direction by Mr. V. S. Tamma. It is found 

 that the increased opalescence of the mixture over 



C 2 



