TRANSACTIONS OF SECTION A. 557 



The subjects under investigation are : — The effect ofthediameter of the cordite, 

 of the charging density, and of the shape of the enclosure. 



The curves of rise and fall of pressure are for each explosion automatically 

 recorded, the high-pressure recorder described at a previous meeting ^ being used 

 for this work. 



Attention was drawn to the dangerous vibrations which are set up when the 

 charge is not uniformly distributed throughout the enclosure. 



3. Granular aiid Sjncular Structure in Solids. By G. T. Beilby. 



In a communication made to the British Association in 1901 - 1 drew atten- 

 tion to certain facts which had apparently escaped the notice of other observers in 

 micro-metallurgy. It was shown that transparence in metals is not only found 

 in such specially attenuated forms as thin leaves or films deposited on glass, but 

 that it is an intrinsic property of the metal even in its more massive forms. It 

 was further shown that metal surfaces under obliquely reflected light exhibit a 

 remarkably uniform granular or spicular structure which appears to be quite distinct 

 from the crystalline structure revealed by the etching methods of micro- 

 metallurgy. 



During the past two years my study of this subject has been continued and ex- 

 tended, and some of the results have been already published.^ 



The object of the present communication is to place on record such confirma- 

 tion and modification of the original observations and statements as have resulted 

 from the further study of the subject. 



The original statements depended on microscopic observations made by obliquely 

 reflected light with objectives of moderate numerical aperture. This form of illu- 

 mination can only be conveniently applied with objectives whose working distance 

 is not less than 5 mm., and whose front lens is not very large. It was therefore 

 desirable that the observations by obliquely reflected light should be supplemented 

 by others in which different methods of illumination could be employed. 



A study was made of films of metal which were thin enough to transmit light 

 freely. By transmitted light, if such films are not too thin, they show a distinct 

 granular texture, as if the substance had been partly gathered up into minute 

 mounds. By alternately illuminating one of these films by transmitted and by 

 obliquely reflected light it is seen that the structure which is granular by one 

 light is spicular by the other ; the spicular appearance, therefore, is caused by a 

 granular texture. The slightness of this texture is shown by the fact that it is 

 visible in oblique light in metal films which are less than 10 fijx in thickness. 



By a parallel study of the surface-layer in metals in their more massive forms 

 it was found that this layer is in many respects distinct from the mass which it 

 covers, being in its structure and properties similar to the thin films deposited on 

 glass. 



The character of the material on which the film is supported has a considerable 

 influence on the appearance by obliquely reflected light. In the case of massive 

 metal the opaque highly reflecting under-surface adds a light and colour to the 

 spicular appearance which is absent in that of the thin glass-supported films. But 

 if due allowance is made for this the correspondence between the appearance of 

 the two, the surface layer and the thin film, is so exact as to leave no doubt as to 

 the identity of the structure which causes this appearance. 



The transparence of thin films of metal was studied by Faraday, and some of 

 his conclusions have been confirmed by subsequent observers. His very remark- 

 able observations on the effect of heat annealing on thin films appear to have 

 dropped out of sight. The subject has been studied by me with the help of lenses 

 of a resolving power much greater than any which could be obtained in Faraday'? 



' See Report, British Association, G\&sgov!,\{)Q\,^. 1(^8, a.n([ Phil Man vo) iii 

 p. 461, 1902. 



■•' Report, 1901, p. 604. ' Pruc. Roy. Soc. vol. Ixxii. No. 481. 



