342 REV. JAMES FARQUHARSON ON THE ICE 



coloured objects seen at the bottom, and they were fringed in every part with spi- 

 culse 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 conclu- 

 sion 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 of 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 

 establish itself, and is not inverted by the mechanical action of a stream. When the 

 temperature of a body of water is under 39°, then the coldest 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 temperature 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. 



M. Arago's illustrations also furnish us with a satisfactory explanation of the 

 curious facts, that the ground-gru makes its first appearance in the more rapid and 

 agitated parts of the stream, and begins to show itself on the bottoms of the stiller 

 parts, and to accumulate there in quantity, only after a longer continuance of the 

 clear frosty weather. In the rapids the hydrostatic order is overturned, and the 

 colder, which is also the lighter, water not only mixed with the warmer below, but, 

 at the whirls of the greatest rapids, brought suddenly, without much mixing, into 

 direct contact with the bottom, cooled still lower than itself by radiation. If the 

 water is at the temperature of 32° Fahr. it can give out no heat to the colder bottom 



* 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 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 be- 

 gins forming, and the hydrostatic order is deranged by the wind. 



