354 Prof. Tyndall on some Physical Properties of Ice. 



Thus, whether we apply heat or pressm-ej lake ice melts with 

 peculiar facility in certain directions. 



32° 

 33° 

 34° 

 35° 

 36° 



39° 

 40° 

 40° 

 40° 



Note from Mr. Faraday. 



My dear Tyndall, 



Have the following remarks, made in reference to the irregular 

 fusibility of ice, to which you drew my attention, any interest 

 to j'ou, or by an occasional bearing on such cases, any value in 

 themselves ? Deal with them as you like. 



Imagine a portion of the water of 

 a lake about to freeze, the surface S 

 being in contact with an atmosphere 

 considerably below 32°, the previous 

 action of which has been to lower the 

 temperature of the whole mass of water, 

 so that the portion below the line M is 37° 

 at 40°, or the maximum density, and 38° 

 the part above at progressive tempera- 

 tures from 40° upwards to 33° ; 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 conducted downwards through the ice ; but the suc- 

 cessive series of temperatures from 33° 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 \y eve 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 30° F., 

 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 M'ater to contain a trace of saline mat- 

 ters 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 eff"ects : they would make that layer of greater specific gra- 

 vity than before, and so give it a tendency to sink into the 

 warmer under layer ; but they would also make it require a 



