29G Mr. C. Tomlinson on the Action of Low Temperatures 



end is to use clean filtered solutions in clean tubes, kept plugged 

 with cotton-wool. 



For example, the sulphates of zinc and magnesia in atomic 

 proportions, with a small quantity of water, are heated in a flask^ 

 boiled and filtered into test-tubes in which they are again heated 

 nearly to boiling, and then plugged and set aside to cool. When 

 cold, one of the tubes is put into a freezing-mixture at about 

 — 10° ; and in about ten minutes or so large tetrahedral crystals 

 beautifully formed begin to grow, as it were, from the side of the 

 tube, and they go on increasing until the whole of the solution 

 is transformed into a solid mass. If the tube be now transferred 

 to snow and water at 32°, the solid melts rapidly and the solu- 

 tion becomes clear as before. If, however, the cotton-wool be 

 removed for a moment, either when the solution is solid or liquid, 

 it crystallizes, in the one case during the melting, and in the 

 other immediately. 



The solidification of the solution must not be regarded as 

 case of freezing, since no ice is formed ; it is rather a case of 

 abnormal crystallization of the saline molecules in combination 

 with the water, and capable of existing only at this low tempera- 

 ture under the defined conditions. I call it a case of abnormal 

 crystallization, because most of the solutions that were tried be- 

 haved in the same manner; that is, they formed tetrahedral 

 crystals at about 0° and melted rapidly at 32°. 



Take another example. A supersaturated solution of the double 

 sulphate of copper and magnesia was reduced to about — 4° F., 

 when tetrahedral crystals formed on the surface, the solid angles 

 growing downwards until the whole of the solution became solid. 

 The deep blue colour had disappeared, and the solid presented 

 different shades of very light blue. When the tube was put into 

 snow and water at 32°, the solid retreated from the sides of the 

 tube, and the clear solution appeared intensely blue in contrast 

 with the nearly white solid. 



Sulphate of zinc and potash-alum in atomic proportions formed 

 tetrahedral crystals at 4°. The triple salt was afterwards crys- 

 tallized in an open dish, and 200 grains of it, boiled with five 

 drachms of water, formed a clear solution, which was filtered into 

 a clean tube, plugged, and left to repose during six days. It was 

 then placed in a freezing-mixture at 0°, when a white powder, 

 probably of a basic sulphate of alumina and anhydrous sulphate 

 of zinc, was thrown down, and on this grew a brilliant white fo- 

 liage resembling ivy, having an exquisite effect. This ivy-leaf 

 pattern seems to me to result from deformations of the tetrahe- 

 dral crystals, as I believe was also the case with a supersaturated 

 solution of the double sulphate of copper and nickel, which at 

 0° formed beautiful feather- shaped crystals. A supersaturated 



