1858.] On Ice of Irregular Fusibility. o75 



40, or the maximum density, and the part above at progressive 

 temperatures from 40 upwards to 32 ; each stratum keeping 

 its place by its relative specific gravity to the rest, and having 

 therefore, in that respect, no tendency to form currents either 

 upwards or downwards. Now generally, if the surface became 

 ice, the water below would go on freezing by the cold con- 

 ducted downwards through the ice ; but the successive series 

 of temperatures from 32 to 40 would always exist in a layer 

 of water contained between the ice and the dense water at 40 

 below M. If the water were pure, no action of the cold would 

 tend to change the places of the particles of the water or cause 

 currents ; because the lower the cold descended, the more 

 firmly would any given particle tend to retain its place above 

 those beneath it : a particle at e, for instance, at 36 Fahr., 

 would, when the cold had frozen what was above it, be cooled 

 sooner and more than any of the particles beneath, and so 

 always retain its upper place as respects them. 



But now, suppose the water to contain a trace of saline 

 matter in solution. As the water at 32 froze, either at the 

 surface or against the bottom of the previously-formed ice, 

 these salts would be expelled ; for the ice first formed (and 

 that always formed, if the proper care be taken to displace 

 the excluded salts) is perfectly free from them, and PURE. 

 The salts so excluded would pass into the layer of water 

 beneath, and there produce two effects : they would make 

 that layer of greater specific gravity than before, and so give 

 it a tendency to sink into the warmer under layer ; but they 

 would also make it require a lower temperature than 32 for 

 congelation ; this it would acquire from the cold ice above, 

 and by that it would become lighter and float, tending to 

 remain uppermost; for it has already been shown that the 

 diminution of temperature below 32 in sea water and solution 

 of salts, is accompanied by the same enlargement of bulk as 

 between 32 and 40 with pure water. The stratum of water, 

 therefore, below the ice, would not of necessity sink because 

 it contained a little more salt than the stratum immediately 

 below it ; and certainly would not if the increase of gravity 

 conferred by the salts was less than the decrease by lowering 

 of temperature. An approximation of the strata between 

 the freezing place and the layer at 40 would occur, i. e. the 



