ON RA DIANT HEA T. 63 
transparent water of the cell exerts no sensible absorption 
on the luminous rays, still it withdraws something from 
the beam, which, when permitted to act, is competent to 
melt the ice. This something is the dark radiation of the 
electric light. Again, I place a slab of pure ice in front 
of the electric lamp: send a luminous beam first through 
our cell of water and then through the ice. By means of 
a lens an image of the slab is cast upon a white screen. 
The team, sifted by the water, lias little power upon the 
ice. But observe what occurs when the water is removed; 
we have here a star and there a star, each star resembling 
a flower of six petals, and growing visibly larger before our 
eyes. As the leaves enlarge, their edges become serrated, 
but there is no deviation from the six-rayed type. We 
have here, in fact, the crystallization of the ice reversed 
by the invisible rays of the electric beam. They take the 
molecules down in this wonderful way, and reveal to us 
the exquisite atomic structure of the substance with which 
Nature every winter roofs our ponds and lakes. 
Numberless effects, apparently anomalous, might be 
adduced in illustration of the action of these lightless rays. 
These two powders, for example, are both white, and un- 
distinguishable from each other by the eye. The luminous 
rays of the sun are unabsorbed by both from such rays 
these powders acquire no heat; still one of them, sugar, is 
heated so highly by the concentrated beam of the electric 
lamp, that it first smokes and then violently inflames, while 
the other substance, salt, is barely warmed at the focus. 
Placing two perfectly transparent liquids in test-tubes at 
the focus, one of them boils in a couple of seconds, while 
the other, in a similar position, is hardly warmed. The 
boiling-point of the first liquid is 78 degrees C., which is 
speedily reached; that of the second liquid is only 48 
degrees C., which is never reached at all. These anomalies 
are entirely due to the unseen element which mingles with 
the luminous rays of the electric beam, and indeed consti- 
tutes 90 per cent, of its calorific power. 
A substance, as many of you know, has been discovered, 
by which these dark rays may be detached from the total 
emission of the electric lamp. This ray-filter is a liquid, 
black as pitch to the luminous, but bright as a diamond to 
the non-luminous, radiation. It mercilessly cuts off the 
former, but allows the latter free transmission. When 
