34 Mr Skinner, Observations on the Minute Structure 



simpler than those obtained from glacier ice a description of 

 them will now be given. 



In Figure 1 a cast of a portion of a column of ice from the 

 Schafioch is shown. Its greatest length is 19 - 5 centimetres and 

 the direction of this length was vertical in the cave. As the cast 

 is a negative all the cavities in the cast are really prominences in 

 the ice 1 . The general appearance has given rise to the terms 

 'prismatic structure' used by Browne 2 and Bonney 3 , and 'honey- 

 comb ice ' used by Lohmann. The crystals lie side by side with 

 their optic axes parallel and at right angles to the surface exposed 

 to the source of cold, just as in pond-ice the axes are normal to 

 the surface of the water. They, therefore, present to a spectator 

 the appearance of an irregular mosaic, like the tops of the columns 

 in the Giant's Causeway. When crystals are formed in this 

 manner, if the water contain any impurities, these will be collected 

 on the outsides of the crystals where they are in contact with one 

 another. As is well known water which contains salts or gases in 

 solution has a lower apparent freezing-point than pure water. 

 We may imagine the freezing of such water to occur thus : from 

 some point six rays spring, forming a star, and on these secondary 

 rays grow. It is pure ice which is forming, and the solution of 

 impurities, which is becoming more concentrated, is thrust away 

 in front. The impure layers, therefore, will solidify in the inter- 

 stitial spaces last, and they will on fusion commence to melt 

 first. 



If heat be supplied to ice formed in this manner so slowly, 

 that the distribution of temperature is almost uniform, the fusion 

 of the interstitial matter will occur before the purer ice and the 

 individual crystals will be separated sometimes wholly, and at 

 other times incompletely, by furrows. This kind of melting is 

 especially favoured in ice-caverns where the temperature during 

 a great part of the year is only a fraction of a degree above zero. 

 The cast (Fig. 1) is that of a case of this kind. The plaster of 

 Paris has flowed into the interstitial spaces to different depths up 

 to 5 millimetres. The same phenomenon is shown by artificial ice 

 if the block be allowed to warm very slowly. It is also shown by 

 many alloys on fusion. 



On the exposed surface of many crystals in the Schafioch 

 a special kind of marking may be seen. The hexagonal base is 

 traversed by a number of lines which are approximately parallel. 



1 A positive may readily be made by pressing putty on tbe cast. But to get the 

 visual appearance of a positive this is unnecessary, as the figure when looked at in 

 a certain way reverses itself and appears to be a positive. I find this optical reversal 

 especially easy with figure 2 when the figure is inverted. Viewed in a certain way 

 a coin which is actually in relief appears to be incised. 



2 Browne, G. F., Ice-Caves of France and Switzerland, 1865. 



3 Bonney, Proc. Camb. Phil. Soc, March 4, 1867. 



