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NA TURE 



[September 23, 1922 



is at or near the zenith, and it dims rapidly with 

 increased obliquity of its rays. The seasonal variation 

 due to this cause' is said to be too often overlooked 

 or underestimated by amateur photographers, the 

 strength of the light "being about twice as great in 

 summer as in late autumn or winter. It is mentioned 

 that during heavy rainfall the light is photographically 

 stronger than in "densely cloudy weather without rain, 

 due to the light reflected from the falling raindrops. 

 Tropical daylight is asserted to be about twice as 

 strong photographically as summer daylight in lati- 

 tude 40°, and about four times as bright as winter 

 daylight at this latitude. The light is much brighter 

 along the sea coast than inland. Chemical activity 

 eloping and fixing processes is greatly increased 

 with high temperatures, and correspondingly retarded 

 with low temperatures. Photographic films and 

 prints may be subjected either to high temperatures 

 or high humidity without excessive deterioration, 

 but not to both 'in combination. Both prints and 

 films are said to deteriorate rapidly in the moist 

 tropics, but those developed and fixed under tropical 

 conditions have a greater permanence in the tropics 

 than those developed and printed in the temperate 

 zone and subsequently taken to the tropics. 



Electrical Resistivity of Steels under Stress. 

 ■ — The recent researches of Bridgman have shown that 

 under hydrostatic pressure the resistivity of steels 

 decreases, while the earlier work of Tomlinson on 

 stretched steel wires showed that under tension less 

 than the elastic limit the resistivity increased. 

 According to the May issue of the Science Reports 

 of Sendai University, Mr. Sin-iti Fukuta has. under 

 the direction of Prof. Honda, carriecTthe observation 

 of the effect of tension on resistivity beyond the 

 elastic limit, and has succeeded in showing that up 

 to stresses of the order of 5000 kilograms per sq. cm., 

 steels with various carbon contents increase in 

 resistivity I -14 io-° per cent, per kilogram per sq. cm. 

 of tensile stress, the proportionality continuing past 

 the shoulder of the stress-strain curve. In all cases 

 about 90 per cent, of the observed change of resistance 

 of the specimen was due to its elongation and cross 

 contraction. 



Glass Research. — Volume iv. of " Experimental 

 Researches and Reports " has recently been pub- 

 lished bv the Glass Technology Department of the 

 University of Sheffield. It comprises a series of reports 

 by Dr. W. E. S. Turner and his staff, principally on 

 the influence of aluminium on sodium and sodium 

 calcium trisilicate glasses. Aluminium is shown in 

 the first paper. No. VII., to facilitate manipulation 

 in lamp-working and to assist in preventing devitrifica- 

 tion. The second paper on the effect of aluminium 

 on the annealing temperature is less convincing, as 

 it neglects questions of time and rates of cooling. 

 Pelouze's conclusion that as aluminium is substituted 

 for sodium the density increases, is reversed in paper 

 IX. ; density and refraction both appear to dimmish. 

 Careful stirring has evidently been necessary to detect 

 the small variations recorded. In determining the 

 thermal expansion effect of silica and sodium oxide 

 in sodium silicate glasses, a silica factor value very 

 different from that of Schott has been obtained. In 

 the next paper, No. XV., the effect of aluminium on 

 thermal expansion is considered, but further research 

 evidently required. Two of the most practical 

 papers deal with the relative advantages and dis- 

 advantages of limestone, burnt lime, and slaked lime 

 in 1 ommon glass batches containing soda ash and salt 

 caki I he shrinkage, porosity, and other properties 

 of British fireclays are discussed in paper XXIII. 



NO. 2760, VOL. I IO] 



Comparison is made with one foreign clay only, the 

 German Grossalmerode. In view of the present 

 conditions it might have been well to include several 

 of the French clays which compare favourably with 

 Grossalmerode. Following two papers on lime- 

 magnesia glasses, a general report on glass and one 

 on the refractory materials, both by Dr. Turner, are 

 reprinted. The glass industry is to be congratulated 

 upon its close association with the University of 

 Sheffield and the Society of Glass Technology. 



Photographic Sensitometry and Testing. — The 

 Washington Government Printing Office has issued 

 No. 439 of the Scientific Papers of the Bureau of 

 Standards on the " Sensitometry of Photographic 

 Emulsions and a Survey of the Characteristics of 

 Plates and Films of American Manufacture," by 

 Raymond Davis and F. M. Walters, jun. For several 

 years the Bureau of Standards has made measure- 

 ments of the characteristics of photographic light- 

 sensitive materials, aiming at uniformity in the 

 standardisation of methods, so that the results by 

 various workers may be directly comparable. The 

 present paper gives details of the principles involved 

 in photographic sensitometry and testing generally 

 as introduced by Hurter and Driffield and published 

 over and over again during the last thirty years. 

 Perhaps it is desirable to restate them to render the 

 paper more complete. The methods of the Bureau 

 are more original. Their light source is a 6 to 8 volt 

 Mazda C automobile headlight with a special blue 

 glass filter, giving 2-73 candle-power and the colour 

 of average yearly noon sunlight at the latitude of 

 Washington. The principal other deviation from 

 H. and D. methods is that the Bureau of Standards 

 defines the speed of a plate as 10 divided by the 

 inertia, instead of 34 divided by the inertia as adopted 

 by Hurter and Driffield to fit in with their actinometer. 

 For colour sensitometry a replica grating is used with 

 a slit 2 inches long, and the exposure is graduated by 

 a disc with suitably curved apertures that is rotated 

 close in front of the slit. The methods of making 

 other tests are fully described. Appended are 86 

 charts, each dealing with a single plate and giving 

 three characteristic curves representing the result of 

 development for 3, 6, and 12 minutes respectively, a 

 contrast development curve, a fog contrast curve, the 

 fog being exclusive of the glass and gelatine, a spectro- 

 gram showing colour sensitiveness, exposure factors 

 for several colour filters, speed, extent of the straight 

 part of the characteristic curve, and the resolving 

 power estimated by a standardised method. Only 

 sensitive materials made in the United States, and 

 practically all of these, are discussed. 



Separation of Isotopes of Lead. — In the 

 Scientific Proceedings of the Royal Dublin Society 

 for August (vol. xvii. N.S. No. 6), Drs. T. Dillon and 

 R. Clarke and Mr. V. M. Hinchy describe some pre- 

 liminary experiments on the separation of the isotopes 

 of lead by a chemical method. The process is based 

 on the reaction between lead chloride and an organo- 

 magnesium compound : 



2PbCl, + 4 MgRX = PbR 4 +2MgCl 2 + 2MgX„ +Pb. 

 Hoffmann and Wolf in 1907 had already found 

 that when lead chloride containing radium-D reacted 

 with magnesium phenyl bromide, most of the radio- 

 activity was found in the metallic lead separated by 

 the above reaction, and this was confirmed. With 

 the two portions of lead separated, the atomic 

 weights 207-1 and 207-3 or 20 7'4 were found, and it 

 is considered that the different isotopes of lead are 

 not identical in their chemical properties in the 

 reaction chosen. Further experiments are in progress. 



