It has been supposed, we think erroneously, that water contains " latent heat," that is, material atoms of heat in chemical combination with it, and that these are given off during the process of congelation. This supposition of " latent heat " is evidently only tenable on a sort of corinwoular theory of heat, for on the undulatory theory of heat " latent heat," would be synonymous with latent motion, and the idea would involve a contradiction. There cannot be latent heat on the undulatory theory, and it remains for us to offer another explanation which shall be consistent with that theory.
The facts are simply these :---During the time which is occupied by the water in changing its form from the liquid to the solid state it does not propagate any undulations of heat in the sumunding space, and thereby become colder. The propagation of undulations of heat by the action of the atoms of water upon those of the ether contained in it is for a time arrested. It is only necessary to stath the fact in the above form. No further explanation is required.
Conversely : if a lump of Wenham Lake ice at a temperature of 10° be brought into a warm room, it does not begin to melt until the temperature has risen to 32°, and it remains stationary at that temperatuth until the whole of the ice is melted ; the temperature of the water then begins to rise. The explanation is the same as before.
" Latent heat " may be a convenient term to employ, but the facts should not be misunderstood.
It would be out of place to pursue this subject further, but all the phenomena of latent heat may be explained in the way we have pointed out.
" Latent light " is another expression which has be,en adopted somewhat hastily in attempting to explain some phenomena of light lately discovered by M. Niepce de St. Victor. Certain substances called " Solar phosphori " have the well known property of becoming self luminous by being exposed to light ; just as a piece of slate has the property of becoming hot by being held before the fire. The piec,e of slate when removed from the fire radiates heat ;—the solar phosphorus, when removed from the sunshine and taken into a dark room, radiates light. The two cases are strictly parallel. This property of solar phosphori has been long known, but M. Niepce
has discovered lately that white paper, and other substances, become actinic, after having been exposed to sunshine, and in this state react upon sensitive photographic tablets in the dark room. These substances have been said to absorb light by insolation, and retain it as " latent light " within their pores ; but this explanation involves the same absurdity as the idea of " latent heat." The fact probably is, that the insolated substance does not convert the actinic rays into heat rays, as bodies mostly do, but propagates them unchanged, when taken into the dark, and thereby produces the effects due to actinism, when laid upon a sensitive photographic paper or plate. The experiment described by M. Niepce, in which light is supposed to be potted in a sealed cylinder, and liberated after any interval of time by removing the cover, may be explained thus :— The inside of a hollow cylinder of tin is lined with paper which has been steeped in a solution of nitrate of uranium. It is then ex posed to sunshine for some time, and sealed up. On removing the sealed cover, in the dark room, some months afterwards, and apply-, ing the open end of the cylinder to a piece of sensitive chloride paper, it is darkened all over ; or if an engraving on india paper be laid upon it, so as to act as a negative placed between it and the contents of the tube, an image of the engraving is obtained. The explanation is, that the nitrate of uranium in the paper is deoxi dized by light, and converted into a proto-nitrate of uranium. This becomes oxidized again in time, and converted into a per-salt, at the expense of the oxygen contained in the aqueous vapour in the tube, thereby liberating hydrogen, which darkens the chloride paper, or prints an image upon it through india paper.
To seal up light in a pot would be the same thing as to seal up motion, for light is motion. Such an idea is surely absurd.
LevENnEn Ram The faintly lumimous rays beyond the violet end of the spectrum are called " lavender rays." They are said to be highly actinic, but the maximum of actinic power lies probably within the violet rays ; this, however, may depend upon the nature of the substance acted on.