NATURAL PHILOSOPHY. 143 



to little granules of ice. The next stage is that the ice becomes 

 black, showing that it is soaked, as it were, with water; and if at 

 this time there is any open water, as where a river falls into a 

 lake, and wind enough to produce a swell, the whole surface of 

 the ice may be observed to undulate. If the ice now breaks up 

 prematurely with a high wind, it becomes a mass of spiculce of 

 ice which have not reached the melting-point, and which I have 

 seen accumulated to the depth of 6 or 7 feet against the 

 edge of the ice which has not yet broken up. But if there is no 

 wind the whole surface of the lake may appear an unbroken sheet 

 of black ice, still a couple of feet thick, till, in an astonishingly 

 short time, sometimes not more than a few minutes, it disappears 

 as if by magic. So sudden is this disappearance that the ice is 

 popularly believed to sink. 



" I once had a very good opportunity of noticing this sudden 

 disappearance. I had built on the ice during the winter a pier of 

 logs tilled with stones, and when the spring came it settled down 

 to the bottom, carrying with it a large cake of the ice. When the 

 lake had opened, I went round the pier in my canoe to see if it 

 had settled evenly. There, at the bottom, in 6 or 7 feet of water, 

 lay the cake of ice it had carried down, with the chips made in 

 building the pier still imbedded in it ; and, as I looked, blocks 

 would break off, of a foot or more in thickness, rise to the surface, 

 and almost instantaneously disappear The true ex- 

 planation of the prismatic structure appears to me to be the lines 

 of air-bubbles. These are visible in all ice before any thaw has 

 commenced, and in the process of freezing they seem to be formed 

 in vertical lines. When the thaw occurs these lines of bubbles 

 form the centres, as it were, from which it penetrates in every 

 direction through the mass." 



o 



REFRACTION AND DISPERSION OP OPAQUE BODIES. 



The results of a research on the relation of the refractive in- 

 dices and dispersion of opaque bodies, published in Poggendorff's 

 " Annales," by Wernicke, are thus given by the author : 



The examination of thin plates giving colors by interference, 

 leads to the conclusion " that all bodies of strong dispersion have 

 optical properties in common, which appear of interest for the 

 theory of light. 



" It is known from experience that dispersion and absorption are 

 related to one another ; and Cauchy, in his ' Memoire sur la dis- 

 persion de la Lumiere,' has given an equation in which this re- 

 lation is implicitly contained. The discussion of this equation, of 

 which one side is an infinite series, presents some difiiculties; it 

 has been thought sufficient to retain the first two terms of this series, 

 and neglect the rest. This would be permissible, as M. Christof- 

 fel has shown, if in every case the sphere of action were infinitely 

 small in comparison with the wave-length. That the last as- 

 sumption is not admissible, however, the discussion of the incom- 

 plete equation shows ; for it yields the result that every spectrum 



