RADIATION OF CALORIC.

101

SECT. xri.

RADIATION

OF

CALORIC.

Caloric is thrown oft' or radiates from heated bodies in rightlines, and moves through space with inconceivable velocity.It is retarded in its passage by atmospheric air, by colourlessfluids, glass, and other transparent bodies.

For if a glass mirror be placed before a fire, the mirror trans-mits the rays of light, but not the rays of heat.

If a plate of glass, talc, or a glass vessel filled with water besuddenly interposed between the fire and the eye, the rays oflight pass through it, but the rays of caloric are considerably re-tarded in its passage; for no heat is perceived until the inter-posed substance is saturated with heat, or has reached its maxi-mum. It then ceases to intercept the rays of caloric, and allowsthem to pass as freely as the rays of light.

It has been lately shewn by Dr. Herschel that the rays of ca-loric are refrangible, but less so than the rays of light; and thesame philosopher has also proved by experiment, that it is notonly the rays of caloric emitted by die sun, which are refrangi-ble, but likewise the rays emitted by common fires, by candles,by heated iron, and even by hot water.

Whether the rays of caloric are differently refracted, in dif-ferent mediums, has not yet been ascertained. We are certain

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