The cleanliness and condition of solid surfaces

Another consequence of these films of grease is the prevention of oriented overgrowths .4 It is often found that if a crystal with a clean, freshly cleaved surface, is moistened with a solution of an isomorphous, soluble salt, which is then allowed to evaporate, the crystals deposited from the soluble salt are oriented with their edges parallel to those of the original crystal. Such overgrowths are well shown by sodium nitrate deposited on calcite. They are only obtained in perfection, however, if the surface of the first crystal is free from contamination the power of orienting deposited isomorphous crystals is much diminished if the freshly cleaved surface is left in the air. 

6 Glass surfaces seem to become dirty more quickly than most others this is very likely due to the contamination having penetrated below the surface into cracks, and coming out gradually after cleaning the actual surface. 

The conductivity of surface films of adsorbed moisture may be a serious trouble in high voltage engineering practice. 

The surface tension, and total surface energy, of solids. Strictly speaking, the work required to produce a fresh surface of a solid varies from atom to atom, and the surface tension is therefore not uniform. It is, however, possible to speak of the surface tension of a solid as the average work necessary to produce one square centimetre of fresh surface if a sufficiently large area of surface is taken, the irregularities will be averaged out. 

At present there is no trustworthy method for measuring the surface tension of most solids. An estimate for mica1 depends on the work required to cleave the crystal. Ostwald s older solubility method involves large and incalculable errors in practice. The usual methods for liquids fail, because of the non-mobility of the surface. 

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