GEOLOGY: 5. TABER 
297 
the race any further and no longer possible to separate two distinct 
races by selection without a cross with normal. 
The conclusions that logically follow from the preceding discussion 
are that (1) extra bristles are primarily occasioned by one germinal unit 
and further influenced by other germinal units, and (2) that no change 
that could have either evolutionary or practical significance has oc- 
curred in these units during the 50 generations of the experiment. 
1 Pearl, R., /. Exp. ZooL, Philadelphia, 13, 1912, (283-394); Nilsson, Hj., see De Vries, 
Plant Breeding; De Vries, H., The Mutation Theory; Tower, W. L., Washington, Pub. Car- 
negie Inst., No. 48; Johannsen, W., Elemente der Exacten Erblichkeitslehre. 
2 Castle, W. E., and Phillips, J. C, Washington, Pub. Carnegie Inst., No. 195; Smith, 
L. H., Univ. III. Agric. Exp. Sta. Bull., No. 128; Middleton, R., /. Exp. ZooL, Philadelphia, 
19, (451-503); Jennings, H. S., Genetics, Cambridge, 1, (407-534). 
^MacDowell, E. C, /. Exp. ZooL, Philadelphia, 19, (61-98). 
* Castle, W. E., and Wright, S., Washington, Pub. Carnegie Inst., No. 241. 
PRESSURE PHENOMENA ACCOMPANYING THE GROWTH OF 
CRYSTALS 
By Stephen Taber 
DEPARTMENT OF GEOLOGY. UNIVERSITY OF SOUTH CAROLINA 
Communicated by J. C. Branner, February 26, 1917 
Under suitable conditions crystals grow in directions in which growth 
is opposed by external force. This fact appears to have been first 
observed by Lavalle in 1853.^ It was denied, however by Kopp, who, 
after making certain experiments, stated that he was never able to ob- 
serve anything tending to confirm the view that a crystal can raise it- 
self in order to grow also on the side on which it rests. ^ Subsequently 
the observations of Lavalle were confirmed by Lehmann^ and others. 
Becker and Day seem to have made the first attempt at determining 
the magnitude of the force developed during crystal growth. In their 
experiment, a crystal of alum supporting a weight was covered with a 
saturated solution of alum, and supersaturation was induced by evap- 
oration. The crystal increased in size through growth on the lateral 
exposed faces which were also extended downward, thus lifting the 
crystal together with its load. The deposition of new material on the 
lower surface was restricted to the periphery, so that a hollow face was 
gradually formed by the downward extension of the new growth, and 
the crystal rested on a very narrow outer rim. The area of this rim was 
determined with difficulty, but repeated measurements led to the con- 
clusion that "the force per unit area which the crystals exert .... 
