4>2 



A^A TURE 



[June i6, 1910 



purposes, hardness rather than ductility is desired, 

 and alloys containing rather more than 4 per cent. 

 of copper, or the corresponding amount of manganese, 

 can be employed. The casting of these alloys pre- 

 sents some difficulty, but a considerable number of 

 founders are able to produce castings of this kind 

 with regularity ; the secret of their success lies largely 

 in casting the alloy at a suitable temperature, and in 

 the preparation of a mould having a hard and very 

 dry surface. All the alloys of this class undergo a 

 comparatively enormous amount of shrinkage in 

 passing from the totally liquid to the totally solid 

 condition, and unless due allowance is made for this 

 contraction, faulty castings always result. 



In the case of the aluminium-zinc alloys, a difficulty 

 of another kind arises ; while these alloys are less 

 viscous when molten, and flow into the moulds more 

 freely than the aluminium-copper alloys, they are very 

 weak and brittle while hot, and castings made of 

 these alloys are very apt to crack while cooling if 

 their contraction is opposed to any considerable ex- 

 tent ; it is probably on this account that these alloys 

 have acquired the reputation of being "treacherous." 

 They have, on the other hand, been employed with 

 some success for the production of so-called "die 

 castings." These castings are produced by means 

 of metallic moulds, and can be made so accurate 

 that no machining is required even for such objects 

 as screw-threads and certain parts of instruments. On 

 the other hand, these alloys are said to be weak under 

 vibration, but this statement as yet requires confirma- 

 tion by systematic investigation. 



The question of the power of light alloys to resist 

 corrosive influences is one of considerable importance; 

 it has been generally accepted by those accustomed 

 to deal with aluminium and its alloys that the pure 

 metal is much more resistant to corrosion than any 

 of its alloys, and, as regards some of these, this view 

 is undoubtedly correct. The numerous " solders " 

 which have been advocated for jointing aluminium 

 and its alloys all suffer very seriously from this point 

 of view. It must, of course, be borne in mind that 

 aluminium itself has a powerful affinity for oxygen, 

 and only protects itself from rapid atmospheric oxida- 

 tion by the formation of a very thin coating of oxide 

 on all exposed surfaces ; if this coating is ruptured, as, 

 for instance, by friction, continuous oxidation re- 

 sults, and the presence of an alloyed element in the 

 form of a distinct constituent may cause such interrup- 

 tion. Again, the contact of aluminium with another 

 metal, in the case of all those metals usually met 

 with in engineering construction, leads to the forma- 

 tion of galvanic couples, and the consequent rapid 

 corrosion of the aluminium. In this way also an 

 alloyed element may intensify corrosion. On the other 

 hand, it is equally possible that the presence of an 

 alloyed metal may improve the protective coating of 

 oxide formed on the surfaces of the metal, and there 

 is good reason to believe that the presence of copper 

 produces this effect to some extent, while the presence 

 of manganese — as has recently been shown — facili- 

 tates the formation of a surface "patina" containing 

 manganese oxide as well as alumina. 



Even in the best circumstances, however, the pro- 

 tection of light alloys from corrosion is a most im- 

 portant matter, and this is accentuated by the difficulty 

 of finding a suitable paint or varnish the constituents 

 of which do not act upon aluminium — an action 

 which generally takes the form of an interchange of 

 oxygen between the pigment and the metal. Pro- 

 cesses for coating the light alloys with a less cor- 

 rodible metal, such as copper, tin, zinc, &c., have 

 been tried, but these modes of protection are accom- 

 panied by the risk of an increased amount of local 

 corrosion owing to galvanic action. If the metallic 



NO. 2120, VOL. 83] 



coating is anywhere broken through. A more hope- 

 ful line of thought is to be found in the development 

 oi processes for coating the alloys with an adherent 

 layer of some inert compound of aluminium, much as 

 iron and steel are coated with a layer of phosphate of 

 iron in the " Coslettising " process. 



Finally, some reference may be made to the possi- 

 bilities of the use of magnesium and its alloys for 

 the production of light and strong materials of con- 

 struction. The fact that magnesium has a specific 

 gravity of only 174 at once suggests its use for such 

 a purpose, but the fundamental objection lies in the 

 fact that it is much more corrodible than aluminium, 

 and that therefore the attainment of even moderate 

 durability in its alloys must be a problem of much 

 difficulty. That some solution of this problem may 

 have been found is suggested by the statement re- 

 cently made that the newest German Zeppelin air- 

 ship is to be constructed of an alloy known as 

 " Elektron," said to be an alloy of aluminium and 

 magnesium. Its density is stated as being close to 

 ry, so that it must clearly consist of magnesium 

 alloyed with only i or 2 per cent, of aluminium. 

 No data as to the strength of such an alloy are 

 available, but from the known constitution of the 

 alloys of the aluminium-magnesium system, it appears 

 probable that the addition of aluminium to mag- 

 nesium in proportions up to 7 or 8 per cent, will 

 materially increase the strength of pure magnesium, 

 but the actual results cannot be predicted; it is, how- 

 ever, probable that pure magnesium is rather weaker 

 than pure aluminium, so that it would be surprising 

 to find in this group an alloy having a density less 

 than i'8, with a tensile strength above 10 or 12 tons 

 per square inch. Alloys of aluminium with small 

 proportions of magnesium are, it may be mentioned, 

 in somewhat extensive use, particularly for certain 

 parts of scientific instruments, under the name of 

 "magnalium," but these alloys, although somewhat 

 lighter, are not so strong as the best of the alumin- 

 ium-copper and aluminium-copper-manganese series. 



From the foregoing review of the question it will 

 be seen that the problem of light alloys is still far 

 from a satisfactory solution, and that there is a need 

 for further systematic study of the alloys of the 

 lighter metals. Walter Rosenhain. 



GREEK ARCHMOLOGY.' 



THE "Annual of the British School at Athens" 

 still remains of the somewhat unwieldy size that 

 it has assumed of late years. A return to the more 

 convenient bulk of, say, vol. viii. would be welcomed 

 by the reader; yet it cannot be said of any of the 

 articles in vol. xiv, that any part of them might 

 profitably have been excised. Only the fourth instal- 

 ment of Dr, Mackenzie's work on "Cretan Palaces" 

 seems rather too long. Still, no doubt the various 

 questions raised by Dr. Dorpfeld's criticism of Dr. 

 Mackenzie's former articles. Dr. Noack's work on 

 Cretan buildings, and the discoveries of Neolithic pro- 

 totypes of the " Homeric " palace in Thessaly, needed 

 exhaustive treatment. So we are compelled to post- 

 pone reading Dr. Mackenzie's views on the relations 

 of the Homeric house to the Cretan palaces until next 

 year. 



The director of the school and his assistants con- 

 tinue their account of the discoveries at Sparta, which 

 have conferred such lustre upon British archaeological 

 work during the last three years. Few believed that 

 excavations at Sparta would prove so interesting. 



1 "The Annual of the British School at Athens." Vol. xiv. (Session. 

 1007-8.) Pp. x + 468 ; 15 plates. (London : Macmillan and Cr., Ltd., 1909 / 

 Price 25J. net. 



