POLYMORPHIC TRANSFORMATIONS OF SOLIDS. 101 



of two papers by Tammann. Reference is made to Tanimann's paper 

 for a historical account of our knowledge of the relation of the two 

 phases. It is to be noticed that my notation is different from that of 

 Tammann. I have employed consistently the numeral I for the high 

 temperature form, whereas here Tammann applies it to the low tem- 

 perature form. The special interest in the pressure effects on this 

 substance is because the forms I and II are of the rather unusual ice 

 type, the form at the higher temperature having the smaller volume. 

 Mallard and Le Chatelier found at 20° and 2475 kgm. a large decrease 

 of volume; one would not expect a transition here from the initial 

 trend of the curve at 1 kgm. This led Tammann to take up the 

 question, and he showed in his first paper that the effects were doubt- 

 less due to a third modification. He was not able to obtain any very 

 great amount of information as to this third modification, however, 

 because of the large amount of lag which the reaction showed. He 

 took up the matter again later, however, and was able to get nuK-h 

 better numerical results. The data of the later paper of Tammann 

 cover about the same range as the present paper as far as the I-II and 

 the II-III curves go. Tammann, however, followed the II-III curve 

 carefully to a temperature 10° lower than that used here, and made 

 measurements connected with the subcooliag, etc., down to the tem- 

 perature of liquid air. The only extension of range in this paper is on 

 the I-III curve, which is followed here to 200°, whereas Tammann 

 did not determine accurately any points on it, but showed only that 

 its slope was positive, and calculated its probable value. Tammann 

 in his second paper devoted a good deal of attention to the question 

 of lag. He found that at 90° the equilibrium pressure could be shut 

 between the limits 2956 and 2957 kgm.; at 60° the limits were 2980 

 and 3040, and at 18°, 2760 and 3026. The limits within which spon- 

 taneous formation of the nuclei took place were considerably wider 

 than this. In these experiments, on the other hand, the effect of 

 lag was found to be much less. As already stated, the greatest width 

 of the band of indifference of the II-III curve is 60 kgm. at 30°. The 

 reason for this difference between our two experiments is not clear; 

 possibly the kerosene had a catalyzing effect on the reaction which the 

 heavy oil used by Tammann did not have, or it may be that this was in 

 some way an effect of the much higher pressure to which the Agl 

 was subjected in these experiments. Before any readings had been 

 taken, the Agl had been subjected to 12000 kgm. at room temperature. 

 It is to be noticed, however, that Tammann in his first set of experi- 

 ments reported in " Kristallisieren und Schmelzen" found a nmch 



