6 g FRAGMENTS OF SCIENCE. 
transparent to the one class of rays, and highly opaque to 
the other. Thus the white powder, which has shown 
itself so powerful an absorber, has been specially selected 
on account of its extreme perviousness to the visible rays, 
and its extreme irnperviousness to the invisible ones; while 
the dark powder was chosen on account of its extreme 
transparency to the invisible, and its extreme opacity to 
the visible rays. In the case of the radiation from our 
fire, about 98 per cent, of the whole emission consists of 
invisible rays; the body, therefore, which was most opaque 
to these triumphed as an absorber, though that body was 
a white one. 
And here it is worth while to consider the manner in 
which we obtain from natural facts what may be called their 
intellectual value. Throughout the processes of Nature 
we have interdependence and harmony; and the main 
value of physics, considered as a mental discipline, consists 
in the tracing out of this interdependence, and the demon- 
stration of this harmony. The outward and visible phe- 
nomena are the counters of the intellect; and our science 
would not be worthy of its name and fame if it halted at 
facts, however practically useful, and neglected the laws 
which accompany and rule the phenomena. Let us en- 
deavor, then, to extract from the experiment of Franklin 
all that it can yield, calling to our aid the knowledge which 
our predecessors have already stored. Let us imagine two 
pieces of cloth of the same texture, the one black and the 
other white, placed upon sunned snow. Fixing our atten- 
tion on the white piece, let us inquire whether there is any 
reason to expect that it will sink in the snow at all. There 
is knowledge at hand which enables us to reply at once in 
the negative. There is, on the contrary, reason to expect 
that, after a sufficient exposure, the bit of cloth will be 
found on an eminence instead of in a hollow; that instead 
of a depression, we shall have a relative elevation of the 
bit of cloth. For, as regards the luminous rays of the 
sun, the cloth and the snow are alike powerless; the one 
cannot be warmed, nor the other melted, by such rays. 
The cloth is white and the snow is white, because their 
confusedly mingled fibers and particles are incompetent to 
absorb the luminous rays. Whether, then, the cloth will 
sink or not depends entirely upon the dark rays of the sun. 
the substance which absorbs these dark rays with the 
