154 BELL SYSTEM TECHNICAL JOURNAL 



comparison of the electron patterns with X-ray and electron patterns 

 obtained from the bulk material (Fig. 2). 



Electron diffraction patterns from alpha-monohydrate formed on 

 silica surfaces were found to vary markedly with pH of the alumi- 

 num-water solution and with the reaction time. From these patterns 

 the following conclusions were drawn. Monohydrate crystals formed 

 after short reaction times (4 hours to 20 hours) were sharply oriented 

 with a particular crystal plane parallel to the silica surface; the indi- 

 vidual crystals were on the average fairly large (from 5 to 10 X 10"'' 

 cm) in directions parallel to the surface, and thin (2 X 10~^ cm or less) 

 normal to the surface. As the reaction time was increased, the crys- 

 tals became, on the average, thicker normal to the surface (but seldom 



Fig. 2 — Electron diffraction pattern obtained by the reflection method from finely 

 pulverized aluminum alpha-monohydrate (AUOa-HoO). 



as thick as 5 X 10~^ cm), and at the same time other crystals of mono- 

 hydrate were formed which were less nearly perfectly oriented although 

 still showing the same strong preference. For long reaction times 

 layers of completely unoriented alpha-monohydrate crystals were 

 sometimes produced. 



In the presence of traces of organic acids oriented soap crystals were 

 formed as a result of the reaction of aluminum and water. These 

 crystals were produced at all pH values. They appeared as scum 

 upon the water surface, and were not readily adsorbed upon silica. 

 This fact proves that the action of aluminum in preventing develop- 

 ment of silicosis cannot be attributed to an aluminum soap. Figure 3 

 exhibits a typical diffraction pattern from oriented crystals of an 

 aluminum soap. 



