July 15, 1922] 



NA TURE 



1916-17, including a high-grade copper ore from a 

 boulder near Port Lockroy on the Neumayer Channel. 

 The description of the varied lavas of Deception 

 Island in both this and Mr. Tyrrell's paper is surely an 

 argument in favour of collation and co-operation. 



The reports of the British Antarctic (" Terra 

 Nova ") Expedition of 1910, published by the British 

 Museum, are concerned, however, with another 

 quadrant of the antarctic region. In the geological 

 series, Nos. 3 and 4, recent and older sedimentary 

 deposits are described by Mr. F. Debenham, from 



his personal observations as geologist to the expedi- 

 tion. Metamorphic rocks are dealt with in No. 5 ; 

 but their relationships in the field are as yet but little 

 known. Dr. A. Smith Woodward's account (No. 2) 

 of " Fish remains from the Upper Old Red Sand- 

 stone of Granite Harbour," including Bothriolepis, 

 provides more definite information. The remoteness 

 and perils of the district excuse the uncorrelated 

 nature of the results, and no doubt also explain the 

 handsome method of their presentation in the yeai 

 1921-22. G. A. J. C. 



Durability of 

 By Dr. James 

 J T is the custom of optical glass manufacturers to 

 •*- issue long lists of types characterised by their 

 optical constants, without much regard to their 

 qualities of durability, which are only occasionally 

 indicated. To the practical computer these lengthy 

 lists are not imposing. Experience has taught him 

 that the number of sufficiently trustworthy types is 

 really very limited, and that only in exceptional 

 circumstances, that fortunately do not frequently 

 arise, ma}- an extension of his list be justified. But 

 while the possibility remains that glass of an unstable 

 kind may be used in the construction of his instru- 

 ments, the optician has cause for anxiety in the 

 knowledge that his reputation as well as the glass 

 may become tarnished. 



The British Scientific Instrument Research Asso- 

 ciation, the function of which is to provide for the 

 industry the leaven of science, has been charged with 

 the investigation of the durability of glass. According 

 to the admirable report 1 that has recently been 

 issued, " the object of the Research was to determine 

 how far it was possible to establish simple tests by 

 which the durability of different types and varieties 

 of optical glass could be quickly ascertained without 

 awaiting the results of experience by actual use over 

 an extended period." To what extent this object 

 has been attained may be realised from the frank con- 

 fession at the conclusion of the report, that "it is 

 not possible to recommend any simple test by which 

 the durability of an optical glass can be determined, 

 with such reliability as to avoid the chance of mis- 

 leading users of the glass in some one or other applica- 

 tion of it." With this pessimistic conclusion it is 

 difficult to agree, as the object has already been 

 attained in the workshop, if it is agreed that it is 

 the reflecting or transmitting qualities of the surface 

 with which the optician is concerned. 



From the report it appears that numerous tests of 

 Continental and British types of optical glass have 



1 Report of an Investigation on the Determination of the Durability of 

 Optical Glass carried out by T. Haigh. Pp. gi+io plates. (British 

 Scientific Instrument Research Association, 26 Russell Square, W.C.I.) 

 7s. 6d. 



Optical Glass. 



Weir French. 

 been examined by the iodoeosin test of Mylius, the 

 autoclave water and steam tests at four and two 

 atmospheres of pressure, and the " dimming " test 

 evolved by the Royal Arsenal Directorate of Chemical 

 Inspection, which co-operated in the research. This 

 dimming test is really an elaboration of the Zschimmer 

 test. The three tests as applied indicate merely to 

 what extent alkali can be dissolved from the surface, 

 and, as is to be expected, the flint glass types appear 

 more durable than the crown types — a conclusion 

 that is misleading, as the report rightly indicates. 

 Our industrial atmosphere unfortunately contains 

 sulphuretted hydrogen, and if in the dimming test an 

 atmosphere more representative of reality had been 

 employed, the flint types would have been placed 

 more nearly in the order accorded to them by 

 Faraday. 



The report confirms the interesting fact, already 

 known in the workshop, that in the glass -polishing 

 process alkali is dissolved from the surface layer, 

 which, with a few exceptions, becomes more durable. 

 Workshop experience shows that a new cloth polisher 

 tested with litmus will usually be found to be slightly 

 acid ; after a few hours of working it will be neutral ; 

 and thereafter it becomes strongly alkaline. A pitch 

 polisher reacts similarly, but it does not retain the 

 dissolved alkali to the same extent. 



The optician is concerned in practice not so much 

 with those so-called optical glasses that are visually 

 affected by the tests referred to, as with those that 

 are labelled as being durable and unaffected. If a 

 well - polished specimen of the most durable crow-n 

 glass be boiled in water at atmospheric pressure for 

 two hours and its reflecting power be then tested 

 by means of a multi-reflection photometer, a loss of 

 1 per cent, per reflection may be detected. After boil- 

 ing for eight hours, the loss will be about 2 per cent., 

 but thereafter the rate diminishes. 



The drastic autoclave tests adopted in the research 

 are not necessary to demonstrate how many of the 

 types included in the optical glass-makers' lists are 

 vitreous substances of but little value and a source 

 of danger to the unwarv. 



"O EPORTS have been received by the Governor 

 -*•*- from Mr. A. A. Reading, of the Bibundi 

 Estate, of a recent volcanic eruption in the Cameroons 

 Mountain, near the coast of Nigeria. Repeated 

 earthquake shocks commencing on February 3 last 

 were followed by an eruption at an altitude of about 

 4000 feet on the north-west side of the mountain, 

 giving rise to a lava stream which flowed down in the 

 direction of the Bibundi Estate, and entered the 

 plantation area on March 3. The lava extended, 

 roughly, one-third of a mile out to sea, and huge 



Volcanic Activity in Nigeria. 



columns of steam were continually ascending. 

 Attempts to photograph the scene failed on account 

 of the dense smoke and ashes. The centre of the group 

 of craters was estimated to be distant 9900 yards from 

 the house at Bibundi Beach on a magnetic bearing 

 of 128 , and the height above sea-level was believed 

 to be about 4150 feet. 



In May the northern stream was still advancing, 

 and threatened to reach the sea along the water- 

 courses on each side of Dollmanshohe. Sometimes 

 there was a loud noise resembling that of a blast 



NO. 



275O, VOL. I io] 



