June 17, 1922] 



NA TURE 



779 



a 1 



t 



the lighter portion of the alloy ; the silver remains 

 pure and free from base metal. Silver may be 

 separated from copper in the cupel by continual 

 addition of lead until it appears in a state of purity. 

 . . . Gold may be purified from silver and copper in 

 two ways. From copper alone it may be refined by 

 the method used to purify silver from copper, namely, 

 cupcllation with addition of lead. If it is desired, 

 sulphur may be added as well ; this burns the copper 

 and the gold remains pure. Gold may be refined 

 from lead by the method used to refine silver from 

 lead. The purification of gold from silver may be 

 carried out in two ways, one by means of minerals 

 and the other by means of salts. The former method 

 is as follows : the gold alloyed with silver is beaten 

 out into thin leaves and these are placed on a bed of 

 a mixture of haematite and salt and covered with 



ore of the same mixture followed by a layer of red 

 y. The whole is then heated in the oven known to 



en of science as the ' refining-fumace,' when the 



ver is absorbed by the earthy matter and the gold 

 ves are left pure, containing nothing but the most 

 refined gold. 



" This operation may also be carried out in a 

 similar way by using alum and salt, or by means of 

 baked clay. The clay is finely powdered and mixed 

 with an equal amount of salt and the two well 

 powdered again. The mixture is then spread in a 

 layer on a layer of red clay. A gold leaf is then added, 

 followed by another layer of the mixture of clay and 

 -alt, and so on until all the gold has been added. A 

 overing layer of clay and sand is then placed on the 

 lop and the whole strongly heated, when the gold is 

 purified and extracted from the silver. . . . This is 

 the process known as ihahira [refinement] by the 

 people of this art. Gold may also be separated from 

 silver in the same way that it is separated from 

 copper. The gold-silver alloy is mixed with a little 

 opper and the mixture fused, with addition of red 

 iilphur from time to time. The gold refines away 

 iiom the silver and is left pure. The former method, 

 however, is the more efficient. 



" The silver which is removed from the gold in the 

 process called shahira may be recovered merely by 

 the addition of mercury to the earthy residue. The 

 mercury thickens and coagulates until it becomes like 

 dough, and this is the sign [of the completion of the 

 action]. When it has become like dough it is placed in 

 a crucible over the fire and the mercury then volatilises 

 away from the silver." E. J. Holmvard. 



Chfton College, May 29, 1922. 



The Notion of Asymmetry, 



MoDERK refinements in our ideas of atomic and 

 molecular structure at once demand a more precise 

 definition of what exactly is meant by molecular 

 asymmetry. Whether this asymmetry be due to 

 certain groupings around a particular atom, or to 

 the structure of the molecule as a whole, such physical 

 properties as optical activity or enantiomorphism must 

 ultimately be shown to be definitely related to the elec- 

 tronic and nuclear arrangements in the molecule itself. 



Langmuir has shown that substances with molecules 

 possessing similar electronic environments closely 

 resemble each other in many of their physical pro- 

 perties, and he calls this phenomenon isosterism, 

 the substances themselves being denoted isosteres. 

 In this brief discussion it will only be necessary to 

 consider the application of this idea to the simplest 

 ase of stereoisomerism. 



The molecule Cubed, where a, h, c, d, are all different 

 itoms or groups, is asymmetric. It exists in two 

 stereoisomeric forms, one the mirror image of the 

 other. The substance having this molecular structure 

 may crystallise in two enantiomorphously related 



NO. 2746, VOL. 109] 



forms, and may rotate the plane of polarisation of 

 light. Now let d be replaced by c' , where c' is an 

 isostere of c. Such a molecule is now no longer 

 asymmetric as regards the arrangement of its electrons, 

 but is certainly asymmetric if we take into account 

 the inner nuclei. If the rotation of the plane of 

 polarisation of light is dependent entirely on electronic 

 movements, such a substance may not be optically 

 active, and it may not even crystallise in two forms. 

 We have not sufficient data to decide this matter ; 

 but it is obvious that the application of this idea 

 to the case of isotopic elements entering into 

 combination might lead to some interesting in- 

 vestigations. We will only consider the substance 

 CH . SO3H . Cl,5 . CI37, where CI3J and CI37 are the 

 two principal isotopes of chlorine. This compound, 

 like Cabcc', is asymmetric as a whole, but so far as 

 its electronic environment is concerned, the two 

 stereo forms are identical. 



Now this particular variety of di-chlor-methane- 

 sulphonic acid must be formed to a certain extent in 

 the ordinary preparation of the substance, and it 

 would seem that its isolation could be effected, only 

 if it did actually exist in two enantiomorphously 

 related forms. From what has been said this seems 

 rather unlikely, but still, an investigation in this 

 direction would throw some light on the matter, 

 one way or the other. Thomas Iredale. 



University College, London. 



The Evolution of Plumage. 



May I be allowed to refer to some of the statements 

 in the article on the Evolution of Plumage published 

 in Nature of May 20, p. 662. 



(i) The writer (H. F. G.) states that in the case 

 of ducks and penguins " the difference between their 

 nestling coat and the final dress is enormous." In 

 my paper on " The Nestling Feathers of the Mallard," 

 I point out that the tail-quill protoptiles consist of a 

 calamus, containing cones, a shaft and a distinct after- 

 shaft, and especially that the barbules at the tip of 

 the shaft in having hook-like cilia are more specialised 

 than in some of the true metaptile feathers of the 

 adult ; hence it follows that feathers of the first nestling 

 coat, instead of being simpler, may be more complex 

 than true feathers of the adult coat. 



(2) It is stated that in the emus the differences 

 between the nestling feathers (protoptiles) and the 

 feathers of the second and later generations " are 

 reduced to a question of mere size." As the figures 

 in my paper clearly show, the aftershaft of the feathers 

 of the first generation is represented by a few simple 

 barbs, whereas the aftershaft in the following genera- 

 tions is as long and as complex as the shaft. 



(3) Owls and petrels are said to " have as thick 

 and fluffy and iong-lasting mesoptile coats as any 

 penguin." In the case of the tawny owl the mesoptile 

 coat is poorly developed and (in specimens I reared) 

 shed soon after growth is completed — perhaps, like 

 Pycraft, the writer of the article regards the feathers 

 forming the first coat of true feathers in the tawny 

 owl as mesoptiles ; the feather figured in " A History 

 of Birds " (p. 270) is not a mesoptile but a true 

 (metaptile) feather. In a young petrel I received 

 last autumn from Dr. Eagle Clarke, the mesoptile 

 coat consists of simple feathers less than half an inch 

 in length — in penguins the mesoptiles are complex and 

 sometimes reach a length of four inches. 



J. C. EWART. 



The University. Edinburgh. 



(i) Surely the coats differ, almost beyond recogni- 

 tion in penguins ; even in ducks, in which the first 

 dress does not consist only of tail-quills. 



