194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



This is the reason that the curves separating the domains of the 

 different kinds of ice could not be followed to lower temperatures 

 than are shown in the diagram. At lower temperatures the reaction 

 becomes so very slow that it would have taken days to obtain a single 

 point. It is to be expected that the curves separating II and V and 

 V from VI will continue to run to lower temperatures, that they will 

 finally meet, and that from the point of intersection a new equilib- 

 rium curve, the curve between II and VI, will start. The point at 

 which any three curves meet in the diagram is called a triple point. 

 It will be noticed from the figure that two curves never meet without 

 a third curve starting from the point of intersection of the other two. 

 This is always true, provided that on two of the curves there is a 

 phase in common ; it may be proved mathematically that such is the 

 case, but to prove this here would take us too far afield. 



The fact that ice I gives place to ice II at a certain pressure has one 

 practical application. We have often heard of the immense pressures 

 developed when water is allowed to freeze in a closed vessel. Burst 

 water pipes are a familiar example of this, and there are also well- 

 known experiments in which cannon balls have been split open by 

 freezing water. It is of interest to inquire how much pressure might 

 be reached in this way. The diagram furnishes an answer to the 

 question, as it shows that if the pressure on the ice during freezing 

 should rise too much over 2,000 or 2xl0 3 kgm., corresponding to 

 30,000 pounds per square inch, the ordinary ice would change to ice 

 III, which has a much less volume, so that the ice would tend to 

 shrink and the rise of pressure would be arrested. Thirty thousand 

 pounds per square inch is, therefore, the highest pressure that can be 

 obtained by freezing water in a closed space. 



A word as to the possibility of proving that the various new forms 

 of ice that have been described are really solids. All that has been 

 shown in the experiments is that at certain pressures and tempera- 

 tures there is a sudden change of volume. This must mean a change 

 of some kind in the molecular structure of the substance, but on what 

 grounds can it be said that the change is a change to solid form? 

 May not there conceivably be two modifications of the liquid ? The 

 answer is, first, that no substance is known which has two modifi- 

 cations of the liquid, but that many are known which have two solid 

 forms. None of our ideas of the molecular structure of solids or of 

 liquids would lead us to think that two liquid forms of a substance 

 are possible. Secondly, Tammann has given direct experimental 

 proof that the two forms of ice, II and III, are really solid. He did 

 this by cooling the cylinder containing the ice to the temperature of 

 liquid air, and then opening the cylinder after pressure had been re- 

 lieved, still keeping the temperature at that of liquid air. Of course, 

 as soon as pressure was relieved, the ice II or III, whichever it hap- 

 pened to be, became unstable, but at this low temperature the reac- 



