360 Prof. J. C. McLennan on the 



Summary. 



It was found possible, by treating with acid prepared 

 surfaces of a natural crystal of copper, to obtain crystal 

 faces which could be used as reflectors in the X-ray spectro- 

 meter. 



The results of the investigations, thus rendered feasible, 

 showed that in a copper crystal the atoms are arranged on a 

 face-centred cubic lattice. This is the close-packed lattice, 

 to which attention has been drawn by Pope and Barlow. 

 The crystal structure is the most simple of any as yet 

 analysed. 



The Cavendish Laboratory, 

 July 16th, 1914. 



XLIII. On the Absorption Spectrum of Zinc Vapour. By 

 J. C. McLennan, Professor of Physics, University of 

 Toronto *. 



[Plate VI.] 



IN a paper by Wood and Guthrie f on the absorption 

 spectra of certain metallic -vapours, the statement is 

 made that with zinc vapour they found no trace of any 

 absorption in the range of the spectrum investigated by 

 them. This region began at about X5200 and, from indi- 

 cations in the paper, did not extend beyond X2150. 



In view of the prediction made by Paschen % and subse- 

 quently confirmed by Wolff §, that the emission spectrum of 

 zinc should include a series of single lines with the first 

 member of the series at X2139'33, it seemed to the writer 

 desirable to look for absorption by zinc vapour in the region 

 below X2150. 



With the object of investigating this point, some pure zinc 

 was placed in a clear fused quartz tube 2 cm. in diameter, 

 which was then highly exhausted and sealed up. This tube 

 was placed before the slit of a quartz spectrograph and heated 

 with a Bunsen burner. When the spark between zinc 

 terminals in air was used as the source of light, it was found 

 that the vapour evolved when the zinc was melted and raised 

 to a red heat was sufficient to completely cut out the line 

 X2139 # 33. With this moderate heating the absorption band 

 was sharply edged and quite narrow. With stronger heating, 



* Communicated by the Author. 



f Wood and Guthrie, Astrophys. Jl. vol. xxix. no, 1, 1909, p. 211. 

 % Paschen, Ann. der Phys. 1909, vol. xxx. p. 746, and 1911, vol. xxxr. 

 p. 860. 



§ Wolff, Ann. der Phys. 1913, vol. xlii. p. 825. 



