368 



NATURE 



[June 29, 19 16 



ites. Hornblendites also occur, but appear not to be 

 associated \vith copper deposits to the same extent 

 as the two first-named. It will be noted that the 

 igneous rocks are of a decidedly basic type and that 

 they are rich in magnesian minerals, although the 

 absence of olivine forms a constant and interesting 

 feature in their mineralogical composition. The 

 igneous intrusions assume many different forms, such 

 as dykes, pipes, sheets, and irregular bodies, but no 

 trt.:e batholites have yet been met with. No fewer 

 than 344 such intrusions have been mapped up to 

 the present. Very many of these rocks show a 

 certain admixture of sulphide, including copper 

 sulphide, in the form of interstitial grains. The 

 question whether these sulphides are or are not 

 original constituents of the rocks does not admit of 

 any very precise answer, but must be decided by a 

 review of the whole of the phenomena characterising 

 these occurrences. Dr. Rogers concludes, upon the 

 whole of the evidence, that the copper deposits are 

 magmatic segregations; that "the intrusions were 

 complex bodies of two or more differentiates from 

 one magma basin"; that each differentiated portion 

 of the magma held a certain quantity of sulphides at 

 the time of intrusion ; and that these sulphides often 

 collected together within the individual differentiates, 

 and that they were further able to migrate and to 

 impregnate the country to a distance of a few feet 

 from the contact. The paper forms an interesting 

 contribution to the study of magmatic ore deposits, 

 a group to which increasing attention has been 

 devoted during recent years. 



The geographical problems in boundary marking 

 are discussed by Sir Thomas H. Holdich in the 

 Geographical Journal for June (vol. xlvii.. No. 6). Sir 

 T. H. Holdich has had a great deal to do with 

 frontier delimitations in India and South America, 

 and no man is better qualified to speak on the sub- 

 ject and to direct attention to the necessity of geo- 

 graphical knowledge on the part of the statesmen 

 who decide frontiers. The paper gives many in- 

 stances of complications, needless expense, and the 

 threat of war due to ignorance of geographical con- 

 ditions or the misapplication of geographical terms. 

 The question will soon be one of vital importance. It 

 may be too much to hopye that expert geographical 

 advice will be sought at least in the wording of fron- 

 tier treaties, but it is nevertheless not an unreasonable 

 demand to make. 



Mr. O. F. Cook gives an interesting account of 

 agriculture and native vegetation in Peru in the 

 Journal of the Washington Academy of Sciences, 

 vol. vi., No. 10, May, 1916. Mr. Cook deals particularly 

 with the region around Cuzco, the chief centre of the 

 Inca and pre-Inca civilisation. He points out that 

 the present distribution of the principal types of vege- 

 tation is not a natural effect of altitudes, climates, 

 or soils, but an artificial result of intensive agricul- 

 tural occupation of land over a long period of time. 

 The primeval forest which probably clothed the hills 

 has, in his opinion, been everywhere destroyed for agri- 

 cultural purposes, and the forests which are now found 

 are of secondary origin, having sprung up on land 

 which has gone out of cultivation. The absence of 

 palms in such forests is cited in support of this view. 

 He considers that the denudation of the higher land 

 formerly under cultivation has given rise to the large 

 areas of grass land now sterile and abandoned. 



The Optical Society, 39 Victoria Street, Westmin- 

 ster, has reprinted in pamphlet form, at the price of 

 a shilling, the paper on the manufacture and testing 

 of prismatic compasses read recently before the Society 



NO. 2435, VOL. 97] 



by Mr. F, E. Smith, of the National Physical Labora- 

 tory. It describes the methods adopted at the labora- 

 tory to test the instruments for the possible errors, 

 and gives sufficient details to enable any maker to set 

 up without great expense his own testing arrange- 

 ments. In addition, much valuable information is 

 given as to the best form of needle, the best shape 

 of the hard steel pivots, the superiority of garnets 

 to agates as jewels, the proper degree of hardness of 

 the needle (secured by the faintest straw colour in 

 tempering), the advantage of magnetising the needles 

 in coils giving a magnetic field of 400, and the supe- 

 riority of a dead-beat motion of the needle, secured by 

 the use of liquid, air, or magnetic damping. In prac- 

 tical use Mr. Smith thinks it advisable to tap the com- 

 pass gently to give the needle the best chance of taking 

 up a correct position. He finds many of the compasses 

 at present made cannot be trusted to half a degree. 



Bulletin No. 59 of the Technological Series of 

 the Bureau of Standards gives an account of an 

 investigation of standard test specimens of zinc-bronze 

 (Cu 88, Sn 10, Zn 2) by C. P. Carr and H. S. Rawdon. 

 The authors conclude (a) that the addition of the 

 small percentage of zinc does not affect the theoretical 

 microstructure of the alloy; (b) that the method of 

 casting, pouring temperature, etc., affect the structure 

 only indirectly by influencing the rate of cooling, 

 amount and distribution of "enclosures," etc.; (c) that 

 the microstructure offers an explanation for the char- 

 acteristic appearance of the tensile bars after testing ; 

 and (d) that of the various microstructural features 

 affecting the physical properties, oxide films must be 

 considered to exert by far the greatest influence. The 

 best type of test bar where the metal is to be cast into 

 sand is the cast-to-size shape, and if the metal is 

 poured anywhere in the range 1270-1120° C. 

 uniformity of tensile strength and ductility are ob- 

 tained. The advantages of the cast-to-size shape are 

 that it is easy to mould and inexpensive to machine 

 into the shape and size required for testing. It is 

 recommended as the form which should be adopted 

 as standard for general foundry practice. 



Considering what a fundamentally important sub- 

 stance it is, and the fact that it is frequently used in 

 molecular weight determinations, one would have 

 thought that trustworthy data for the melting and 

 solidifying points of benzene would have been recorded 

 long ago. From an article by Mr. R. Meldrum in 

 the Chemical News for June 9, however, this does 

 not seem to be the case. With the most nearly pure 

 benzene commercially procurable, which was solidified 

 at 3° C. for twenty-four hours, and then drained, this 

 author obtained 3-92° and 3-95° C. as the solidifying 

 point. The rate of crystallisation at this temperature, 

 however, slackened very considerably after 10 per 

 cent, had solidified. For the melting point, determined 

 by keeping the thermometer immersed in the melting 

 crystals, the value obtained was 4° C. Using the 

 crystals solidified from the sample, after pressing be- 

 tween filter paper at 3° C, the author found 56° for 

 the solidifying point and 57° for the melting point. 

 Benzene kept in a tube of i-in. bore at a temperature 

 of 1° C. solidified without crystalline structure, and 

 hence was probably in the colloidal condition. Mr. 

 Meldrum concludes that above the melting point ben- 

 zene exists in more than one modification. 



There has just been completed on one of the main 

 lines of the Great Central Railway a bridge over one 

 of the English rivers (which cannot be named for 

 military reasons) having a Scherzer rolling-lift open- 

 ing span weighing 2900 tons, the largest in this 

 country, if not, indeed, in the world. An illustrated 



