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THE RADIATION AND CONDUCTION OF HEAT. 
No meteorological phenomena are so pregnant with interest, or of such pre- 
eminent import, to the gardener, as the transit of heat from one substance to 
another, and from terrestrial bodies to the atmosphere. And yet, speaking gene- 
rally, there is scarcely a cultivator who could clearly explain their nature, or 
definitively state how they are accomplished. Many most ridiculous notions 
concerning them are rife in the world of horticulture, a few of which have even 
been supposed to receive confirmation from professedly scientific sources ; so that 
it is incumbent on some one to place the subject in so simple a light that no further 
mistakes may be justifiable. This task we propose now to attempt. 
Philosophers have long since determined that changes of temperature, in 
common with all other procedures of Nature, are regulated by certain unvarying 
laws. Of these, one of the most prominent is the transition of heat from a warm 
body to any cold one that may be contiguous, till the temperature is equalized. 
Now, although air is too seldom regarded as a refined and subtile substance, such 
is its unquestionable nature. Hence, the process termed radiation is, in point of 
fact, though to a limited extent, a kind of conduction ; the small particles of 
matter of which the atmosphere is composed being the media through which, when 
brought into contact with objects on the earth's surface, their heat is abstracted. 
The above position does not, however, wholly hold good ; since heat is said to 
be capable of pervading a vacuum, and matter cannot consequently be a necessary 
auxiliary to its dispersion. For all practical purposes, therefore, and likewise to 
facilitate the inculcation of the precepts of science, a very proper distinction is 
made between radiation and conduction. Both are the result of expansion, — one 
of the most striking properties of heat ; but the former is the means through which 
the temperature of a body is lowered by diffusing itself into a colder air, while 
the latter term is applied to the passage of heat from a warm solid substance to a 
cold one, when placed in immediate contact therewith. 
Radiation, as the very word implies, is the divergence of a number of heated 
atoms, in the form of rays, from a body thoroughly warmed ; or, as some assert, 
it is the mere emission of calorific rays, causing a greater or less undulation in the 
constituents of the atmosphere, whereby an increase of temperature is occasioned. 
Thus, the sun radiates heat perpetually, some of its rays being transmitted through 
our air to the earth, by which they are received, and from which, during the 
absence of that brilliant luminary, they again emanate in a similar manner. It is 
very frequently confounded with refraction, which is quite another process, and 
most markedly different ; radiation being the simple issue of heat from any surface, 
and refraction the interception of its rays by an interposed screen, and their 
re-radiation from thence towards the point from whence they originally proceeded. 
For example, the earth refracts many of the rays that reach it from the sun, and 
