RAY 
lionour of Leonhard RauwolfF, physician at 
Augsburg, a genus of the Pentandria Mono- 
gynia class and order. Natural order of 
Contorte. Apocineae, Jussieu. Essential 
character : contorted ; berry succulent, two- 
seeded. ' There are four species. 
RAY, in optics, a beam of light, emitted 
from a radiant, or luminous body. Rays 
are defined, by Sir Isaac Newton, to be the 
least parts of light, whether successive in 
the same line, or contemporary in several 
lines. For that light consists of parts of 
both kinds is evident, since one may stop 
what comes this moment in any point, and 
let pass that which comes presently after : 
now the least light, or part of light, which 
may be thus stopped, he calls a ray of light. 
Rays of the Sun. It has been found by 
experiment, that there is a very great diffe- 
rence in the heating power of the different 
rays of light. 
It appears, from the experiments of Dr. 
Herschel, that this heating power increases 
from the middle of the spectrum to the red 
ray, and is greatest beyond it, where the 
rays are invisible. Hence it is inferred that 
the rays of light and caloric nearly accom- 
pany each other, and that the latter are in 
different proportions in the different colour- 
ed rays. They are easily separated from 
each other, as when the sun’s rays are trans- 
mitted through a transparent body, the 
rays of light pass on seemingly undimiiiish- 
ed, but the rays of caloric are intercepted. 
When the sun’s rays are directed to an 
opaque body, the rays of light are reflected, 
and the rays of caloric are absorbed and 
retained. This is the case with the light of 
the moon, which, however much it be con- 
centrated, gives no indication of being ac- 
companied with heat. It has also been 
shown that the different rays of light pro- 
duce different chemical effects on the me- 
tallic salts and oxides. These effects in- 
crease on the opposite direction of the 
spectrum, from the heating power of the 
rays. ' From the middle of the spectrum, 
towards the violet end, they become more 
powerful, and produce the greatest effect 
beyond the visible rays. From these dis- 
coveries it appears that the solar rays are 
of three kinds: 1. Rays which produce 
heat; 2. Rays which produce colour; and, 
3. Rays which deprive metallic substances 
of their oxygen. The first set of rays is in 
greatest abundance, or are most powerful 
towards the red end of the spectrum, and 
are least refracted. The second set, or 
those which illuminate objects, are most 
REA 
powerful in the middle of the spectrum. 
And, the third set produce the greatest 
effect towards the violet end, where the 
rays are most refracted. The solar rays 
pass through transparent bodies without in- 
creasing their temperature. The atmo- 
sphere, for instance, receives no increase of 
temperature by transmitting the sun’s rays, 
till these rays are reflected from other 
bodies, or are communicated to it by bodies 
which have absorbed them. This is also 
proved by the sun’s rays being transmitted 
through convex lenses, producing a high 
degree of temperature when they are con- 
centrated, but giving no increase of tem- 
perature to the glass itself. By this method 
the heat which proceeds from the sun can 
be greatly increased. Indeed, the inten- 
sity of temperature produced in this way is 
equal to that of the hottest furnace. This 
is done, either by reflecting the sun’s rays 
from a concave polislied mirror, or by con- 
centrating or collecting them by the refrac- 
tive power of convex lenses, and directing 
the rays thus concentrated on the combus- 
tible body, 
REACTION, in physiology, the resist- 
ance made by all bodies to the action or im- 
pulse of others, that endeavour to change its 
state whether of motion or rest. 
REALGAR, in chemistry. Arsenic, mi- 
neralised by sulphur, forms two ores, named 
orpiment and realgar, the chemical distinc- 
tion of which is not very accurately deter- 
mined. That which has been named realgar 
is of a red colour, sometimes inclining to 
scarlet, sometimes to orange. It occurs 
massive, disseminated, and crystallised, in 
oblique, tetraedral, or hexaedral prisms, 
generally small and translucent, or semi- 
transparent, with a shining lustre. Its frac- 
ture is uneven : it is soft and brittle, and 
has a specific gravity of 3.2, or 3.3. It ex- 
hales before the blow-pipe a white arsenical 
smoke, with an arsenical and sulphurous 
odour, and gives a blue flame. It consists 
of arsenic and sulphur in the proportions of 
80 of the former, and 20 of the latter. 
REASONING, the exercise of the fa- 
culty of the mind called reasoning ; or it is 
an act or operation of the mind, deducing 
some unknown proposition from other pre- 
vious ones that are evident and known. 
REAUMURIA, in botany, so named in 
honour of Ren6 Antoine Ferchault de Reau- 
mur, a genus of the Polyandria Pentagynia 
class and order. Natural order of Succu- 
lentae. Ficoide®, Jussieu. Essential cha- 
racter : calyx six-leaved ; petals five j cap- 
