Purification of Gallium. • 393 



The fully-developed branches are invariably thinnest at the 

 point of attachment to the " trunk " and grow more massive 

 at the outer or free ends. (In figure 3e the tips of the wires — 

 on which the branches were mounted for convenience in pho- 

 tographing — can be seen at the free ends of the branches and 

 should not be confused with the details of the gallium 

 structure.) 



The growth of a tree takes place in the following stages. It 

 begins as a spherical ball which appears to be perfectly smooth 

 at first but which becomes visibly rough as the diameter 

 increases. These rugosities develop into sharp points which 

 eventually build up a configuration similar to the outside of a 

 chestnut burr. After this the characteristics of a figure of 

 revolution are gradually lost as certain branches grow more 

 rapidly than others. In general, the branches of young trees 

 are thin and pointed while those of more mature trees are 

 bludgeon-shaped as shown in figure 3. Under the given 

 experimental conditions about 20 hours were required to 

 deposit the structures reproduced in the photographs. When 

 the cathode consists of a bare platinum wire which projects 6 or 

 8 millimeters below the free surface of the electrolyte, the trees 

 develop relatively broad leaves instead of roughly cylindrical 

 branches. These leaves are approximately horizontal and are 

 piled up one above another over the entire length of the sub- 

 merged portion of the wire. 



Passing from geometrical to physical and chemical proper- 

 ties, the following facts may merit recording. The material 

 of the trees is hard and strong like the solid phase derived in 

 any other manner. Hence the trees are stable, permanent 

 structures so long as they are kept at a temperature ten 

 degrees, more or less, below the melting point. For this 

 reason, as well as on account of their general appearance and 

 ease of control during development, the gallium trees would 

 afford a more interesting and instructive lecture experiment 

 than the classic lead ones. Throughout the period of electro- 

 lytic deposition the trees are intensely black. This color is 

 largely superficial, for when the branches and trunk are exit to 

 pieces, the freshly exposed surfaces have either the dull appear- 

 ance of slightly tarnished lead or the characteristic silver luster 

 of clean gallium. After removal from the electrolyte and 

 drying, the branches become partially covered with gray and 

 white patches, as may be seen in the photographs. Three 

 trees which had been kept under water for over a month and 

 then allowed to dry were chiefly grayish black. The ends of 

 their branches, however, had turned brown and a pure white 

 powder completely filled the spaces on the undersides of the 

 trees where the branches joined the trunk. The color changes 



