1891.] On the Plasticity of an fee Crystal. 337 



We first notice that the plasticity exists down to 14'4. At this 

 emperature the bending was slow, but this was due in great part to 

 the fact that it came at the beginning, and the bar was as usual. The 

 rapid growth of plasticity, independently of the temperature, is shown 

 by the rate of 0'59 mm. per hour at a mean temperature of 5'6, 

 being raised in less than two hours to 1*18 mm. at 6'l. The tend- 

 ency to recover when the weight is removed is shown three times over 

 in the table. As might be expected, it soon becomes very slow, and 

 in that case after twelve hours, when the recovery amounts to 0'72 

 mm., it has probably stopped altogether. In the fall of rate from 

 1-89 at -17 to 1-18 at -6'l and O745 at -13, in spite of the 

 natural tendency for the rate to rise, we seem to have a real effect of 

 temperature. After 8.38, the cigar box had to be left open as the pointer 

 had almost reached the lid of the box, and so the subsequent tem- 

 peratures are unreliable. I imagine that the change from Ofc85 to 

 0*745 was due to a fall of temperature. 



At the beginning of Exp. 11 the bar measured 14 mm. by 12'3 mm., 

 which was reduced at the end of Exp. 12 to 13 mm. by 10'8 mm. 

 The evaporation had been rather more rapid just at the bend of the 

 bar. This was owing, I believe, to the circulation of air through the 

 ole by which the string passed out. 



I measured the total depression on the trace as 2'6 mm. As mea- 



red by the pointer it is 2'45. The agreement is as good as could be 

 expected. 



Exp. 13. In this experiment I used a thicker bar and tried a 

 variety of weights. The bar was only just small enough to go into 

 the stirrup. (See Table IV, next page.) 



The stiffness of the bar in the first three hours is surprising. 



Exp. 14. In all the experiments hitherto on bars composed of 

 single crystals it happened that the optic axis had been vertical when 

 the ice was formed, so that the planes of freezing coincided with the 

 sliding layers. I fully believed that this coincidence was merely acci- 

 dental, and what happened in Exp. 8 had confirmed this idea, but I 

 thought it desirable to have a more direct proof. So I cut a piece 

 out of a good large crystal in the ice, found on the surface of the 

 water in the bucket, in which the optic axis was not Vertical. When 

 the bar was put in position the planes of free/ing were vertical and 

 parallel to the length, and the optic axis was normal to the length 

 and inclined at about 50 to the vertical. The bar was about 8 mm. 

 square, and the distance between the supports was 51 mm. 



Under a weight of 0*62 kilo, in 4 hours 28 minutes at a mean tem- 

 perature of 4 0> 4 (the maximum 1'4) it bent downwards about 

 4 mm. There was a large lateral bend, which made the vertical bend 



Sy difficult to measure. 

 1 the sliding layers had been necessarily the same as the planes of 

 



