BRIDGMAN. — WATER UNDER PRESSURE. 527 



needless, since Tammann has already given direct proof that II and III 

 are solid. 



The most striking evidence of Tammann is for III. Tammann cooled 

 a steel cylinder containing ice III to the temperature of liquid air, then 

 released the pressure and took ice III out and examined it. The tem- 

 perature was so low that the reaction from III to I did not run imme- 

 diately. Ill was solid and gradually changed to ice I with large 

 increase of volume. In another experiment Tammann placed an elec- 

 tric contact maker in the water. Freezing to a solid was shown by the 

 refusal of the contact maker to work, both for III and II. 



That V and VI ai-e also solid is made probable by two bits of evi- 

 dence. In the first place, the reactions, between V and III and V and 

 VI are exactly similar to those between I and III and I and II. There 

 is no mistaking the difference between a reaction solid-solid and liquid- 

 solid. In the second place, when the reaction. VI-V runs with 

 increase of volume, the containing vessel, whether a glass bulb or a 

 fairly heavy metal cylinder, may easily be destroyed. In this reaction 

 V has the larger volume. It, then, must be^solid, and probably VI also. 



The possible crystalline forms of the different modifications seem to 

 be impossible of direct observation. It would be possible to insert 

 windows in the apparatus and look at all the forms, but this would 

 probably give no information whatever. On several occasions the 

 apparatus was opened and the cylinder of ice I, which had frozen under 

 pressure, was removed. This was always a perfectly structureless, 

 translucent mass, capable of giving no information whatever as to any 

 of its crystalline properties. 



Other Possible Forms of Ice. 



In a recent paper Tammann i^ has stated the probable existence of 

 a fourth variety of ice, very much like ordinary ice in its properties. 

 The evidence for this, was very meagre. Slight discrepancies found on 

 the equilibrium curve I-L could be explained by it ; also a momentary 

 rise of pressure on one occasion after the fall of pressure proper to 

 melting had begun. Seven successive attempts of Tammann to get 

 the same effect again failed. It is evident that the discrepancies on 

 the I-L curve might be due to pressure errors ; a new manometer used 

 by Tammann gave pressures 50 kgm. lower than the former one, and 

 we have already seen that the crossing of the I-III and the I-II curves 

 found by Tammann must be set down to pressure errors. Finally, 

 Tammann states that ice IV may separate out from water when cooled 



" Tammann, ZS. Phys. Chcm., 72, 609-631 (1910). 



