398 SCIENCE PROGRESS 



empty. The hydrogen has apparently a larger diameter than 

 that of oxygen, if we give to oxygen the value of i -30 ; but, 

 if the arrangement is correct, we can truthfully say only that 

 the sum of the diameter of oxygen and hydrogen is 2-76 A.U. 



The recently published work (Iron and Steel Institute, 

 Annual Meeting, May 1922) of Dr. Arne Westgren and Gosta 

 Phragmen, of Stockholm, has considerably cleared the air with 

 regard to the crystalline structures of the various forms of iron 

 and steel. One of these authors presented a report last year 

 {Journal of the Iron and Steel Institute, 1921, No. i, p. 303), 

 but their later paper announces great improvements in their 

 methods and results. They have used the powder method of 

 analysis, the X-rays being produced by a tube of the Siegbahn 

 type, the metallic body of which was constructed of S.K.F. 

 ball-bearing steel. The characteristic rays from an anticathode 

 of iron passed out of five windows simultaneously, while the 

 tube was evacuated by means of a combination of a mercury 

 vapour jet pump and a mercury diffusion pump of the Volmer 

 type. The final arrangement was such that the tube ran very 

 steadily at 45,000 to 50,000 volts and 10 to 12 milliamperes. 

 The reconstructed camera enabled the authors to allow the 

 glowing iron wire to rotate and thus to obtain photographs 

 clear enough for reproduction. The iron wire in the camera 

 was heated by an alternating current of low voltage and the 

 temperature was determined by use of an optical pyrometer. 



In the case of the iron modifications, photographs were 

 taken at ordinary temperature and at about 800° C, 1,100° C, 

 and 1,425° C. The time of exposure was 2| hours. The 

 very interesting result is obtained that, allowing for the slight 

 displacement of the lines due to heat expansion, the photo- 

 graphs of a- and /S-iron are of the same type, while the 7-iron 

 photograph is of quite another type. Again, an examina- 

 tion of the S-iron photograph shows that S-iron also has the 

 same structure as a-iron. Thus the strange fact is revealed 

 that, although it has been considered that iron has four allo- 

 tropic modifications, the transition points of which are about 

 750° C. {at^)y 900° C. (y8;t7), and 1,400° C. (7?:^), yet it 

 exists in two crystalline forms only, the body-centred cubic 

 lattice for a~, ^-, and S-iron, and the face-centred cubic 

 lattice for 7-iron. In other words, the transformation which 

 takes place at 900° C. is reversed at 1,400° C. The funda- 

 mental (unit) cell of the body-centred cubic lattice is a simple 

 cube having one point at each corner and one in the centre. 

 The unit cell of the face-centred cubic lattice is the simple cube, 

 having one point at each corner and one point at the centre of 

 each face. For a-iron at ordinary temperature the side of 

 the unit cube is 2-87 A.U. At 800° C. it has increased to 



