CHEMISTRY OF SOLIDS DESCH 241 



On the basis of these results, an ingenious theory of the action of 

 reagents on solid solutions has been constructed, and although the 

 accuracy of the experimentally determined limits is not high, and 

 there are several exceptions to the rules, an interesting case has been 

 made out. 



If our knowledge of the chemical properties of the interior of a 

 crystal be very incomplete, what are we to say of its surface? Of 

 this we know still less. Even in a crystal of a pure metal there must 

 be some difference in the structure at the immediate surface, on ac- 

 count of the unsymmetrical forces between the atoms in the outer- 

 most layer and its neighbors. For so far as the radius of sensible 

 atomic forces extends, therefore, there must be a condition different 

 from that which prevails at a depth below the surface. One conse- 

 quence is that the surface has residual affinity, which shows itself 

 in the ease with which foreign atoms or ions will attach themselves 

 to it. That the forces acting are chemical is shown by the great 

 effect on the extent of adsorption of the chemical character of the 

 solid and of the adsorbed substance. Films, often one atom thick, 

 attach themselves to the solid, and are only removed with the great- 

 est difficulty. Their presence makes the investigation of the prop- 

 erties of a surface difficult, as the surface actually examined may 

 be in reality quite different from that which is assiuned to be pres- 

 ent. In photochemical experiments with mercury it is usual to pre- 

 jjare a completely fresh surface of the liquid metal by causing it to 

 flow continuously in a fountain, but this device can not be applied 

 to solids. Only rarely can experiments be made with perfectly de- 

 fined solid surfaces. Films of metal prepared by sublimation or 

 sputtering in a vacuum are probably the most under control, but 

 other surfaces are commonly covered by invisible films. 



Schumacher has recently shown that mercury wets glass and silica 

 more and more readily as care is taken to remove hlms from them, 

 and the property of not being wetted by mercury is probably not 

 one of glass and silica, but of those substances coated with a film 

 of gas. Metals most readily take up atoms of oxygen or other ele- 

 ments, forming persistent films, which play an important part in the 

 phenomena of resistance to corrosion. 



There is one way of preparing a fresh surface of a crystalline 

 solid for examination, and that is by cleavage. A freshly cleaved 

 plate of a mineral may be supposed to be clean at the moment of its 

 formation, although it will rapidly take up foreign atoms from the 

 surrounding gas. Tammann has made the interesting observation 

 that a fresh surface of mica is more soluble in water than an older 

 one. Washing with water immediately after cleaving extracts a 

 quantity of alkali salts which is much above the normal solubility of 

 mica, and later washings extract only the normal quantity. It is sug- 



