OPTICS. 109 
The animal heat is constantly different from that of the surrounding air ; 
in the temperate and frigid regions it is generally warmer, in the tropical 
colder than the air. The animal body consequently possesses an 
independent heat which is continually renewed. The heat of the human 
body appears to be nearly the same in all parts of the body, and under the most 
diverse circumstances (cases of disease excepted), varies only from 96.5° 
to 102° F. The animal heat of birds is higher than that of any other 
animal, and that of insects perhaps lower. The source of animal heat is a 
peculiar combustion taking place in the body between the oxygen inspired 
through the lungs and the carbon of the body. Other causes there are, in 
all probability, but this is unquestionably the principal. 
Heat may also be produced by mechanical means. Thus a very 
considerable elevation of temperature may be produced by the compression 
of air, as shown in the little apparatus for inflaming tinder, where the 
tinder fixed underneath an air-tight piston, is inflamed by the sudden 
depression of the piston and consequent compression of the air. Even in 
solid bodies a considerable compression, as in coining money, produces a 
sensible elevation of temperature. Finally, friction is the principal source 
of the mechanical production of heat, for the pivots of a wheel become 
heated to redness if the friction be not diminished by some anti- 
attritient. Wood also may be set on fire by rubbing two pieces 
together. 
OPTICS ; OR, THE THEORY OF LIGHT. 
a. Propagation of Light. 
Bodies are divided, with regard to light, into luminous and non-luminous, 
of which the former emit light peculiar to themselves, while the latter do 
not. Now, luminous bodies are again divided into transparent, or those 
which transmit light; and opake, or those which totally intercept its 
passage. Light is propagated in perfectly uniform media, in straight lines; 
and in curved when the medium is not of this character. In passing from 
one transparent medium to another, it experiences a deviation or break in 
its path; that is, the rays of light undergo refraction.” This, for instance, is 
very evident in its passage from water into air. Take a vessel, v 
(pl. 21, fig. 1), and place in the bottom of it a piece of money. Assume 
such a position with regard to this vessel, that the money shall be just 
concealed by the edge, b, of the vessel. Full this with water, and the coin 
will appear as if elevated, and in plain sight. It appears to lie at n, though 
its position is not changed in the slightest degree; the illusion is produced 
by the bending of the ray, mio, coming from the object to the eye at o. 
Upon this same principle is explained the fact, that the stars are visible 
before their real rising, and after they have actually set. See Astronomy, 
section 47. 
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