FUSION AND SOLIDIFICATION 71 



composition of the residual liquid, there can be 

 no change in the freezing-point. The mixture 

 of salt and water of this particular concentration 

 will solidify completely at a constant temperature 

 into a mixture of salt and ice of the same com- 

 position. But pure chemical elements like lead, 

 or pure compounds like water, also fuse and 

 solidify at constant temperatures without change 

 of composition. In these respects, then, the 

 particular mixture of salt and water which we 

 are considering behaves like a pure element or 

 compound. For this reason Guthrie, who first 

 systematically examined such mixtures, classed 

 them as compounds, and named them cryo- 

 hydrates. It is, however, now evident that their 

 properties are explicable in other ways. 



The phenomena we have traced, and the 

 existence of a cryohydric point must be borne 

 in mind if we wish to understand the structure of 

 natural ice, the properties of metallic alloys, or 

 the processes which occur when, in the cold of 

 an Arctic winter, sea-water becomes coated with 

 a solid covering. 



Natural waters, even when known as fresh, 

 contain some amount of solids in solution. When 

 such waters are cooled to the freezing-point, how- 

 ever, the crystals which appear form the ice of 

 pure water. As the crystals grow, the dissolved 

 salts become concentrated into the liquid which 

 remains ; and the freezing-point of this liquid 

 falls as its concentration rises. Unless the 

 temperature of the cryohydric point is reached, 

 some liquid must always remain, though, with 

 fairly pure water, it may exist only as a thin film 

 between the solid crystals. If the temperature 



