THE BOOK OF THE LANTERN. 197 



These experiments are of great use in demonstrating 1 

 the gradual crystallisation of the various mineral sub- 

 stances of which the crust of the earth is composed. A 

 very beautiful experiment, showing the structure of ice, 

 has been devised by Professor Tyndall. I cannot do- 

 better than describe the manner of performing it in his 

 own words : " Take a slab of lake ice and place it in- the 

 path of a concentrated sunbeam. Watch the track of the 

 beam through the ice. Part of the beam is stopped; part 

 of it goes through ; the former produces internal lique- 

 faction, the latter has no effect whatever upon the' ice. 

 But the liquefaction is not uniformly diffused. From 

 separate spots of the ice little shining points are seen to 

 sparkle forth. Every one of those points is sur rounded by 

 a beautiful liquid flower with six petals. 



" Ice and water are so optically alike that unless the light 

 fall properly upon these flowers, you cannot see them. 

 But what is the central spot ? A vacuum. Ice swims on 

 water because, bulk for bulk, it is lighter than water ; so 

 that when ice is melted it shrinks in size. Can the liquid 

 flowers then occupy the whole space of the ice melted ? 

 Plainly no. A little empty space is formed with the 

 flowers, and this space, or rather its surface, shines in the 

 sun with the lustre of burnished silver. 



" In all cases the flowers are formed parallel to the sur- 

 face of freezing. They are formed when the sun shines 

 upon the ice of every lake ; sometimes in myriads, and so 

 small as to require a magnifying glass to see them. They 

 are always attainable, but their beauty is often marred by 

 internal defects of the ice. Even one portion of the same 



