THE VARIATION OF ANGLES OBSERVED IN CRYSTALS. 
469 
(5.) The Octahedron Angle of Potash-ai.um. 
A large number of observations were next made njjon potash-alnm with the ol)ject 
of discovering whether the octahedron angle of a crystal of this substance changes 
during the growth of the crystal, and if so, in what manner and to what extent. 
Ihe observations were carried out in the following way : a brilliant and uniform 
crystal having been selected, it was mounted in the platinum clip and adjusted on 
the instrument. The glass trough, filled with concentrated solution, and having a 
few small crystals in the bottom of the vessel, w^as then raised until the 
mounted crystal was completely immersed. Generally, the crystal would become 
slightly re-dissolved immediately after immersion, but it Acas constantly watched 
through the telescope until its faces began to grow in a uniform manner, and yielded 
brilliant reflections. It was then measured again and again at short intervals during 
many hours or at longer intervals during several days and nights. 
The best sets of measurements were tliose obtained by immersing the ciystal in 
slightly warmed and supersaturated solution and then (deserving it while the 
temperature was falling for several consecutive hours near an open window during a 
wiiiter night. With a rise of temperature on the following day the crystal would 
become slightly etched and re-dissolved ; but this made it possible to trace the 
changes of angle produced by incipient re-solution, and then tlie plienomena of 
growth could be watched a second time upon the same crystal during the following 
night. 
After once the crystal has growii sufficiently to envelope the platinum clip 
completely, there is no danger of its moving even to the smallest extent during any 
number of subsequent observations ; it may be etched, partly re-dissolved, and allowed 
to grow again many times in succession without any change in its orientation. 
When a crystal of potash-alum was watched in this way It was found :— 
(1) That the images from the various faces continually altered their positions ; 
(2) That the angle between the adjacent faces was never the theoretical angle of 
the regular octahedron ; 
(3) that the faces usually yielded multiple images lying very close together. 
Thus, in the very first crystal measured, the following readings were obtained from 
two very good faces, A being a cube face and o an octahedron face. 
'o 181° IQl', 
|_A 126° 251'. 
oA = 54° 44f' (theoretical angle, 54° 44' 8"), 
