SECTIONAL TRANSACTIONS.— A. 335 



cubic crystals with different compositions. This is possible because the p 

 boundary on the nickel-rich side is curved like a bow. The two-phase 

 (P + pg) area, between the bow and the bowstring, includes the composition 

 FcjNiAl. On slow cooling, this alloy splits up into Feg^NiAl (P) and 

 Fe6Ni7Al7 (P2), with lattice dimensions differing by 0-3 per cent. 



To develop maximum coercivity, the alloy is cooled at such a rate that 

 there is not time to form two distinct types of crystal, but there is a 

 continuous variation of composition. Tiny ' islands ' of iron-rich material 

 are forced to conform to the dimensions of the parent lattice. The 

 resulting strain produces a powerful coercive force. 



Mr. R. W. Powell and Mr. W. A. Wood. — Recent work on mica : 

 thermal and X-ray (12.0). 



(i) A description is given of an adaptation of the divided bar method 

 for determinations of thermal conductivities of insulating materials at 

 elevated temperatures, and of its application to the effect of temperatures 

 up to 600° C. on the thermal conductivity of mica normal to the cleavage 

 plane. Certain micas of the phlogopite type are found to show a sudden 

 decrease in thermal conductivity at about 200° C. with a partial recovery 

 only on cooling. Other specimens are free from this anomaly. 



(2) An X-ray investigation has been made of the variations exhibited 

 by the structure of the same specimens during a temperature cycle of 16° C. 

 to 400° C. It is found that the heat treatment may bring about a very marked 

 mosaic formation without apparent change in external appearance ; that 

 such a structure is not final but may revert partially to the original state on 

 cooling ; and that a close parallelism exists between this effect and the 

 abnormal fall in thermal conductivity. 



(3) The combination of the thermal and X-ray work thus provides a 

 specific illustration of a fruitful application of X-rays to the study of a material 

 of industrial importance. 



General Discussion on X-rays and industry (12.15). 



Afternoon. 

 Visit to Ericsson's Telephone Works, Beeston. 



Friday, September 3. 



Presidential Address by Dr. G. W. C. Kaye, O.B.E., on Noise and the 

 nation. {Illustrated by experiments.) 



Mr. E. R. Davies. — Some recent discoveries on the action of light on photo- 

 graphic materials (11.30). 



The sensitive layer in photographic materials consists chiefly of micro- 

 scopic crystals of silver halide embedded in gelatine. The effect of light 

 is visible only when the exposure is prolonged, metallic silver being formed. 

 Far smaller exposures produce latent images which may be developed — 

 a process of chemical reduction, whereby sufficiently exposed crystals are 

 reduced to silver. This is a type of trigger action since nearly all the energy 

 is supplied chemically and subsequent to exposure. The optical density 

 of the developed image changes with the illumination for constant exposure 



