104 ROYAL SOCIETY OF CANADA 



admission of the radiation of lieat through the water, and therefore 

 goes to support the justness of the theory. The tufts of water star- 

 wort, in the deepest and stillest parts of one of the pools, were the 

 darkest-coloured objects seen at the bottom, and they were fringed in 

 every part with spiculse of gru, at a time while it yet occupied little 

 of the bottom of this pool. The experiments of Boyle, Franklin, 

 Rumford, Leslie (although he denies the conclusion himself), Davy, 

 and Stark appear too uniform in their results to leave any doubt 

 remaining, that dark-coloured bodies both absorb and radiate heat more 

 freely than those which are light-coloured. It is in consistency, then, 

 with an ascertained law of the radiation of heat, that the very dark- 

 coloured tufts of the water starwort should have been the first bodies 

 in the pool cooled to a very low temperature, and, of course, first covered 

 with gru. 



In arguing the whole question, let us not forget to assign a 

 proper value to the illustrations of M. Arago. The first ol them 

 suggests a ready and satisfactory answer to one of the objections 

 which he brings against the theory of radiation, which is, that the 

 effect of it should be as readily manifested in still as in running water, 

 and yet no one has seen a piece of still water frozen at the bottom.^ 



In still water, that hydrostatic order, which M. Arago has so well 

 illustrated as belonging to water when reduced to a temperature under 

 39° Fahr., has free play to astablish itself, and is not inverted by the 

 3nechanical action of the stream. "When the temperature of a body 

 of water is under 39°, then the eoldest portions of it are the lightest 

 and naturally rise and float on the surface. When in a still pond 

 the water nearest the bottom has been cooled below the general tem- 

 perature by contact with the solid materials cooled by radiation, it is 

 displaced by the heavier warmer water above. Hence ice forms first 

 on the surface by the meeting there of both the cold of radiation and 

 that acquired by contact with the incumbent cold atmosphere. 



^ There is an exception to the universality of this position, which, 

 although rare, I have sometimes witnessed; and as the phenomenon is in 

 accordance with the theory of the radiation of the heat from the bottom, it 

 deserves notice. In little ponds of a foot or two deep, dug to obtain the 

 materials for building or agricultural purposes, of which there are many 

 examples in this neighbourhood, after they have been covered, owing to 

 hard and long-continued frost, by a thick sheet of ice, that is sometimes 

 nearly melted off, and the remaining fragments driven to the lee side by 

 a strong westerly gale of high temperature. Such a gale in this climate, 

 frequently, towards its conclusion, shifts to N.W., when the temperature 

 of the air falls again below the freezing-point of water, with a generally 

 clear sky. In such peculiar circumstances the little ponds are suddenly 

 filled with gru, commencing at, and shooting up from the bottom. The 

 whole water is here at 32° Fahr. when the gru begins forming, and the 

 hydrostatic order is deranged by the wind. 



