APPLICATION OF ELECTRON DIFFRACTION 153 



sten ribbon, the distances and the quantities of gold and silica having 

 been adjusted so that the resulting composite film consisted of a layer 

 of silica of thickness 2 X 10~^ cm lying upon a layer of gold of thickness 

 30 X 10~^ cm. This composite film was large enough to supply a 

 great many samples of silica which could be used in a large number of 

 experiments. Each sample was prepared, as and when required, by 

 stripping from the glass slide a small piece of the composite film, dis- 

 solving the gold in a nitric-hydrochloric acid mixture, and then washing 

 the remaining tiny silica film in several changes of distilled water. 



Films prepared in this manner were floated upon distilled vv^ater con- 

 taining aluminum powder, for various lengths of time and at two difi^er- 



Fig. 1 — Electron diffraction patterns from a relatively thick layer of oriented 

 aluminum alpha-monohydrate crystals formed upon a silica film as a result of ex- 

 posure of the film to metallic aluminum and water at i?>° C. A — Electron beam 

 normal to film surface. B — Beam inclined 45° to film surface. 



ent temperatures. In some experiments the pH of the water was 

 adjusted by the addition of HCl or various salts. 



Films treated at 38° C. (approximately body temperature), and at 

 medium and high pH values ^ (6 to 9), gave sharp electron diffraction 

 patterns which were identified with oriented crystals of that hydrated 

 oxide of alumina known as aluminum alpha-monohydrate (Boehmite). 

 Typical patterns are reproduced in Fig. 1. At a low pH value (pH 4) 

 monohydrate crystals were not discovered even after long reaction 

 times. Although the crystal structure of aluminum alpha-mono- 

 hydrate is not known it was possible to make the identification by 



^ The term pH is defined as the logarithm of the reciprocal of hydrogen ion con- 

 centration, hydrogen ion concentration being expressed for purposes of this definition 

 in terms of grams of hydrogen ions in a liter (or more strictly 1000 grams) of solution. 

 In a neutral solution pH = 7; in acid pH < 7 and in alkali pH > 7. 



