238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



must, from the ordinary conception of tin atomic space lattice, 

 be different, the habit of the crystal, that is, the relative development 

 of different faces, will be altered by the presence of a foreign sub- 

 stance. It is on these lines that an explanation of differences of 

 habit must be sought. 



When a face of a crystal is brought into contact with an etching 

 reagent, such as Avater for rock salt, hydrofluoric acid for quartz, 

 or cupric ammonium cliloiidc for iron, the surface is not dissolved 

 away evenly, leaving it suiootli, but characteristic etching pits are 

 produced, the sides of the pits being evidently crystal faces. This 

 shows that chemical action proceeds more readily along certain planes 

 of a crystal than along others, a fact which we should expect from 

 the general properties of the space lattice. It is not explained, 

 however, why these etching pits should appear at first separate 

 from one another, the intervening i)ortions of the surface being un- 

 attacked. Minute particles of some impurity, causing local elec- 

 ti'olytic differences, suggest themselves as a possible cause, but it is 

 unlikely that they w^ould be so evenl}^ scattered in, for example, 

 a quartz crystal as to produce the regular distribution which is 

 often observed. Minute inequalities of level, which may be of a 

 periodic character, are more probable, and this suggestion is strength- 

 ened by the observation that a polished face of rock salt dissolves 

 evenly in water, whilst a natural cleavage face shows etching pits. 



It is now possible, when pursuing the study of solids, to eliminate 

 one of the disturbing factors, the inter-crystalline boundary, by 

 making experiments with specimens composed of a single crystal. 

 There are several ways of preparing single metallic crystals of 

 such a size as to allow of the determination of their physical and 

 mechanical properties. Even so brittle a metal as zinc has an extra- 

 ordinary ductility in single crystals. The mechanism of deforma- 

 tion has been examined in detail by means of X-rays. There is now 

 a large body of evidence as to the directions of slip in a crystal 

 during deformation, and this knowledge is essential to any under- 

 standing of the nature of cohesion, with which the chemical proper- 

 ties are no doubt closely connected. 



We may now turn to the subject of cliemical reactions which take 

 place in the interior of a solid, either originating at the surface 

 or from nuclei which make a si)ontaneous apj^earance in the course 

 of cooling below the melting i3oint. A chemical change which has 

 begun at some point in or at the surface of a homogeneous crystal- 

 line mass cannot advance unless the atoms are able in some way to 

 change their places. Gross movements, represented in gases and 

 liquids by convection currents, are out of the question, but the slower 

 process of diffusion, by which atoms or molecules can make their 



