394 SCIENCE PROGRESS 



merely by the use of sealing-wax. There is no filament in 

 the cathode, the requisite degree of exhaustion being attained 

 by means of a Gaede pump. The great advantage of a tube 

 of this kind, besides the one mentioned above, is the ease 

 with which it can be taken to pieces and put together again. 

 The use of sealing-wax joints allows one to overhaul or substi- 

 tute other metals for anticathode in a very short time. 



The tube used in Sir William Bragg's laboratory at Univer- 

 sity College, London, was designed by Mr. G. Shearer, working 

 for the Department of Scientific and Industrial Research 

 {Electrical Review, vol. xc, June 1922, p. 909). 



Its body is a glass tube about 20 cm. long and 5 cm. 

 diameter (R, Fig. 3). It is not a gas-tube like Miiller's, but 

 uses a tungsten filament, the rays from which are focused 

 by aid of a long brass tube, S, extending practically the whole 

 length of the body, on to a water-cooled anticathode. The 

 joints are of sealing-wax and, with the electrodes, are water- 

 cooled also. The rays emerge from two diametrically opposite 

 windows, W, of aluminium leaf placed at right angles to the 

 anticathode, so that two photographs can be taken simul- 

 taneously. The system is evacuated by a Gaede pump or 

 a pair of Volmer diffusion pumps aided by a weaker subsidiary 

 pump. Under w^orking conditions, the distance from filament 

 to anticathode is from 2\ to 4 cm., from anticathode to 

 powder about 2 cm., and from powder to plate 2 to 4 cm. 

 The crystal powder is either mounted on a piece of paper or 

 contained in a glass capillary tube. In the latter case it can 

 be readily rotated while the photograph is being taken, thus 

 ensuring greater intensity and definition for the lines. 

 Mr. Shearer's tube is generally run at about 30,000 volts, and 

 an exposure of about 6 to 10 milliampere-hours is sufficient 

 to obtain good photographs, especially in the case of organic 

 crystals where the atoms are light. Drawings of naphtha- 

 lene and phthalic acid are shown in Fig. 4. 



The X-ray tube described above can be very effectively 

 used in combination with the ionisation-spectrometer, for 

 Sir William Bragg {Proc. Phys. Soc, vol. xxxiii, June 1921) has 

 now shown that finely powdered crystals can be examined by 

 the ionisation method after the manner of examining a single 

 perfect crystal. The powder is pasted on to a flat surface and 

 placed on the table of the spectrometer in the position ordin- 

 arily occupied by the face of a single crystal. With a bulb 

 current of i milliampere and slits wide enough to take in a 

 reflection over half a degree, the whole spectrum can be run 

 through very rapidly and, if the tube is steady, the relative 

 intensities of the various orders of reflection examined in detail. 

 Spectra of copper, naphthalene, a-naphthol, and benzoic 



