36 
Two opinions are current respecting the nature 
of heat. By one School it is conceived to be 
a peculiar subtile fluid, of which the particles 
repel each other, but have a strong attraction for 
the particles of other matter. By another it is con- 
sidered as a motion or vibration of the particles of 
matter, which is supposed to differ in velocity in 
different cases, and thus to produce the different 
degrees of temperature. Whatever decision be 
ultimately made respecting these opinions, it is 
certain that there is matter moving in the space 
between us and the heavenly bodies capable of 
communicating heat ; the motions of which are 
rectilinear : thus the solar rays produce heat in 
acting on the surface of the earth. The experi- 
ments of Sir W. Herschel have shewn that the 
calorific effects of the solar rays bear no relation 
to their illuminating powers, the red rays pro- 
ducing a much greater effect of heat than any of 
the other coloured rays ; and it appears that there 
are invisible rays distinguished by very different 
degrees of refrangibility, some of which produce 
heat, and others of which are distinguished by their 
chemical effects. 
The different influence of the different solar rays 
on vegetation have not yet been studied ; but it is 
certain that the rays exercise an influence indepen- 
dent of the heat they produce. Thus plants kept 
in the dark in a hot-house grow luxuriantly, but 
they never gain their natural colours ; their leaves 
are white or pale, and their juices watery and 
peculiarly saccharine. 
The earth, when not exposed to the solar rays, 
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