RADIATION AND ABSORPTION. 67 
is upon this principle also that he has indicated, in a general 
manner, that the temperature of space ought to be very little in- 
ferior to the temperature of the poles of the earth, and about 50° 
or 60° below zero; expressing by this valuation nothing else 
than that the total heat which reaches the earth from all the 
celestial bodies, excepting the sun, is equivalent in quantity to 
that which would be emitted on the globe of the earth by an in- 
closure at a maximum emissive power, the sides of which should 
be kept at the temperature of 50° or 60° below that of melt- 
ing ice. The essential point in this manner of regarding the 
phenomena is the possibility of substituting for the whole of 
the celestial bodies a fictitious inclosure, or an athermanous sur- 
face kept throughout at a certain temperature; it remains to 
examine whether there are experiments by which this tempera- 
ture itself can be determined, and with what degree of approxi- 
mation we may hope to obtain it. 
Considered in relation to its nature, the heat of space gives 
rise to a multitude of questions which it would be useless to treat 
of here ; we shall limit ourselves therefore to some observations 
inherent to our subject. We remark, first, that if the fictitious 
inclosure which has just been spoken of can, when a proper 
temperature is assigned to it, represent strictly, or very nearly, 
the heat of space, it can only represent it for its quantity; and 
it will never represent it for its nature, since the heat of space 
possesses essentially properties due to its origin, which it could 
not derive, without doubt, from a source the temperature of 
which should be lower than melting ice. We instantly see that 
from this there result conditions which it is impossible for us to 
reproduce in our experiments, namely, a heat which is by its 
quantity as if it emanated from a cold source, and by its nature 
as if it emanated from a hot source. To explain this kind of 
contradiction it suffices to admit that any line, starting from the 
earth and prolonged indefinitely in space, will not essentially 
encounter a body which can transmit heat to the earth; or, in 
other words, it is sufficient to admit that the planetary inclo- 
sure, in reality, is not for us an inclosure becoming continuous 
by the assemblage of numberless bodies which are dispersed 
in the depths of space at distances continually increasing ; then, 
in fact, there will be points or small portions of the celestial vault 
which would transmit heat to us, and other portions doubtless 
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