March 23, 1905J 



NA TURE 



501 



thus defeating the object of the method. A satisfactory 

 polishing medium for this and other purposes where surface 

 erosion is undesirable was found in calcined oxide of 

 magnesium, the magnesia powder being used in the same 

 way as rouge. 



The section, when polished by means of magnesia, is 

 not yet ready for examination, as it is found that a con- 

 siderable amount of metal is smeared or dragged over the 

 surface, more or less obliterating the true boundary line 

 which it is desired to examine. To overcome this obstacle, 

 it is arranged that the last rubbing on emery paper shall 

 be done in a direction approximately parallel to the 

 boundary of the two metals; the direction of rubbing 

 during the final polishing should then be at right-angles 

 to the boundary, the unavoidable tendency to drag or 

 smear then being such as to draw the iron over the copper 

 on the side where the boundary is to be examined. 



The film of metal smeared over the boundary in these 

 circumstances is extremely thin, and can be removed by 

 slight etching with picric acid. This treatment leaves a 

 clearly defined boundary line appearing under a certain 

 incidence of " vertical " illumination as a narrow black 

 line, and under other illumination being visible merely by 

 the colour-contrast between the iron and copper. 



When a previously polished and etched specimen of iron 

 which has had slip-bands developed upon its surface by 

 strain is treated and examined in this way, the boundary 

 line shows well-marked steps or serrations, readily visible 

 under a magnification of looo diameters. To show that 

 these steps were not due to any of the processes gone 

 through by the specimen, such as the initial etching of the 

 prepared surface or the electro-deposition itself, a series of 

 test specimens was prepared and treated in a similar 

 manner, except that either the preliminary etching, or the 

 deformation, or both, were omitted. The stepped boundary 

 was always found in specimens where slip-bands had been 

 produced, but not otherwise. 



The author therefore feels justified in concluding that the 

 steps seen in transverse sections of strained specimens are 

 the sectional views of slip-bands. It will be seen that the 

 steps, although very minute, are perfectly distinctive, and 

 that they could not be mistaken for generally rounded 

 foldings of the surface ; they possess, in fact, a general 

 geometrical character, which the author regards as con- 

 clusive evidence that they are caused by slip on cleavage or 

 gliding planes of the crystals, and not by any folding or 

 crumpling of the metal. 



" The Effects of Momentary Stresses in Metals." By 

 Prof. Bertram Hopkinson. Communicated by Prof. Ewing, 

 F.R.S. 



If a wire be hung from a firm and massive support, and 

 if a falling weight strike a stop at the lower end of the 

 wire, with a velocity V, it is easy to calculate the strain 

 at any point in the wire at any subsequent time, if it be 

 assumed to be perfectly elastic. When the weight strikes, 

 a wave of extension starts up the wire and travels with a 

 velocity rj=vE/p, where E is Young's modulus, and p is 

 the density. For steel oMs about 17,000 foot-seconds. When 

 the wave reaches the top end, it is reflected down the wire. 

 The history of the strain at any point of the wire is as 

 follows : — When the wave reaches it, the strain, which was 

 zero, suddenly becomes V/a ; it then diminishes as the wave 

 passes over it, according to an exponential law, until the 

 reflected wave reaches it, when it again increases by V/fl. 

 Each bit of the wire is, therefore, subjected to strain which 

 rises suddenly, and then very rapidly diminishes. The 

 maximum strain at any time or place occurs at the top of 

 the wire, where it is i\ ja at the moment when the wave 

 arrives there. For a height of fall of lo feet, and an iron 

 wire, aV/a is 0003, and the corresponding stress is about 

 42 tons per square inch, so that momentary strains greatly 

 exceeding the elastic limit may be produced in this way. 



In the experiments described in the paper, the momentary 

 extension in the top 20 inches of the wire, produced by 

 a blow, was measured by electrical means, and compared 

 with that given by the elastic theory. Where the two 

 agree, and not much permanent extension is left, it follows 

 that the theory is correctl> applied, and that the material 

 is substantially elastic up to the maximum stress, so cal- 



NO. 1847, VOL. 7 l] 



culated, if applied for the time given by the theory. In 

 this way it is proved that a metal wire will stand a load, 

 momentarily exceeding that which (steadily applied) would 

 break it, with but very small permanent extension. In the 

 case of the iron wire, the elastic limit was 17.8 tons per 

 square inch, and the breaking stress 28. 5 tons ; and it was 

 found that a load reaching 333 tons, and exceeding the 

 elastic limit for i/iooo sec, produced very little permanent 

 extension. Similar results were found for copper wire. 



February 23. — " On a New Rhabdosphere." By George 

 Murray, F.R.S. 



The author refers to the interest which the rhabdospheres 

 and coccospheres possess, not only to naturalists, but to 

 geologists and students of deep-sea deposits. He names it 

 R. Blackmaniana, after Mr. V. H. Blackman, his fellow 

 author in an exhaustive study of such organisms (PhU. 

 Trans., B., vol. cxc, 1898). It was obtained by Mr. Murray 

 on the outward voyage to the Cape of the Discovery, 

 in lat. 28° 25' S., long. 23° 56' W., and differs from the 

 only other forms, two in number, known to science in 

 possessing tapering, acute, spinous processes in contrast 

 to the trumpet-shaped and club-shaped processes of the two 

 known species. No sign of the new form has yet been 

 detected in the deep-sea deposits or geological formations, 

 Mr. Murray accounting for this by the minuteness and 

 extreme tenuity of the spines. 



March 2. — " Further Researches on the Temperature 

 Classification of Stars, No. 2." By Sir Norman Lockyer, 

 K.C.B., F.R.S. 



The paper contains a discussion of the more recent photo- 

 graphs obtained with a calcite-quartz prismatic camera. 

 Each negative contained the spectra of two stars, obtained 

 under identical conditions of altitude, exposure and de- 

 velopment, the relative temperatures of which were 

 estimated by comparing the relative intensities of^ their 

 ultra-violet and their red radiations. The term " tem- 

 perature " is understood to include the possible effects of 

 electrical variations. In a previous paper, communicated to 

 the society in February, 1904, the author showed that by 

 thus comparing the relative temperatures of those stellar 

 genera which were placed on different levels of the 

 chemical classification temperature curve, their arrange- 

 ment on that curve was vindicated. In the recent re- 

 search the relative temperatures of the genera placed on the 

 same horizons, but on the opposite sides, of the curve were 

 similarly compared, with the result that their equalitv of 

 tempera'ture, as suggested by the chemical classification, 

 was confirmed. The results also indicate that specific 

 differences exist which will necessitate the subdivision of the 

 previously proposed " genera " into " species." 



Entomological Society, March i.— Mr. F. Merrifield, 

 president, in the chair.— £.^;nbi(ioi!S.—(i) An example of 

 Oxypoda sericea, Heer, taken in Dulwich Wood, June 17, 

 1904, a species new to Britain; (2) O. nigrina, Wat., with 

 a type lent by Mr. E. A. Waterhouse, to demonstrate that 

 it is not synonymous with sericea as stated on the Con- 

 tinent ; (3)' O. exigua, which is also regarded there as 

 synonymous with nigrina : H. St. J. Donisthorpe.— Series 

 of Colias edusa, with var. helice, bred from one 9 helice. 

 sent by Dr. T. A. Chapman from the South of France, 

 to show the proportion of type and variety obtained : H. 

 Main and A. Harrison. The results of similar experi- 

 ments with Amphidasys betularia, bred from a c? var. 

 doubledayaria, and a type ? taken in cop. at Woodford, 

 Essex, in 1903, were also shown. — Specimen of Helops 

 striata, showing an abnormal formation of the right an- 

 tenna, which was divided into two branches from the fifth 

 joint:' R. Priske. — (i) Examples of Hydrotaea pilipes. 

 Stein, (S and 9, the latter sex being previously unknown; 

 (2) several specimens of Hydrotaea tubercidata, Rond, not 

 hitherto recorded as British, captured in various localities ; 

 P. H. Grimshaw. — Cocoons, and perfect imagines of 

 hybrid Saturniids, including 9 and c? of S. pavonia, L.. 

 X S. pyri, Scheff., with added specimens of both sexes of 

 the parent forms for comparison, the cross product re- 

 sembling a large S. pavonia rather than a small S. pyri. 

 The exhibit further included three c? d and three 9 ? , of 

 which the 9 parent was S. pavonia, and the S parent a 



