334 Prof. Tyndall on some Physical Properties of Ice. 



of solar heat upon ice. The experiments were made with the 

 Wenham Lake and Norway ice. Slabs were formed of the sub- 

 stance, varying from one to several inches in thickness, and these 

 were placed in the path of a beam rendered convergent by a 

 double convex lens 4 inches in diameter, and possessing a focal 

 distance of 10^ inches. The slabs were usually so placed, that 

 the focus of parallel rays fell within the ice. Having first found 

 the position of the focus in air, the lens was screened ; the ice 

 was then placed in position, the screen was removed, and the 

 effect was watched through an ordinary pocket lens. 



2. A plate of ice an inch thick, with parallel sides, was first 

 examined : on removing the screen the transparent mass was 

 crossed by the sunbeams, and the path of the rays through it 

 was instantly studded by a great number of little luminous spots, 

 produced at the moment and resembling shining air-bubbles. 

 When the beam was sent through the edge of the plate, so that 

 it traversed a considerable thickness of the ice, the path of the 

 beam could be traced by those brilliant spots, as it is by the 

 floating motes in a dark room. 



3. In lake ice the planes of freezing are easily recognized by 

 the stratified appearance which the distribution of the air-bubbles 

 gives to the mass. A cube was cut from a perfectly transparent 

 portion of the ice, and the solar beam was sent through the cube 

 in three rectangular directions successively. One was perpen- 

 dicular to the plane of freezing, and the other two parallel to it. 

 The bright bubbles were formed in the ice in all three cases. 



4. When the surfaces perpendicular to the planes of freezing 

 were examined by a lens, after exposure to the light, they were 

 found to be cut up by innumerable small parallel fissures, with 

 here and there minute spurs shooting from them, which gave 

 the fissures, in some cases, a feathery appearance. When the 

 portions of the ice traversed by the beam were examined parallel 

 to the surface of freezing, a very beautiful appearance revealed 

 itself. Allowing the light from a window to fall upon the ice at 

 a suitable incidence, the interior of the mass was found filled with 

 little flower-shaped figures. Each flower had six petals ; and at 

 its centre was a bright spot, which shone with more than me- 

 tallic brilliancy. The petals were manifestly composed of water, 

 and were consequently dim, their visibility depending on the 

 small difference of refrangibihty between ice at 32° F. and water 

 at the same temperature. 



5. For a long time I found the relation between the planes of 

 these flowers and the planes of freezing perfectly constant. They 

 were always parallel to each other. The development of the 

 flowers was independent of the direction in which the beam tra- 

 versed the ice. Hence, when an irregular mass of transpai'ent 



