47 
the Theory of Light and Colours. 
Journal. Vol. II. p. 399. ) But the affections of heat may perhaps 
hereafter be rendered more intelligible to us ; at present, it seems 
highly probable that light differs from heat only in the frequency 
of its undulations or vibrations ; those undulations which are 
within certain limits, with respect to frequency, being capable of 
affecting the optic nerve, and constituting light; and those which 
are slower, and probably stronger, constituting heat only ; that 
light and heat occur to us, each in two predicaments, the vibratory 
or permanent, and the undulatory or transient state; vibratory 
light being the minute motion of ignited bodies, or of solar phos- 
phori, and undulatory or radiant light the motion of the ethereal 
medium excited by these vibrations; vibratory heat being a motion 
to which all material substances are liable, and which is more or 
less permanent ; and undulatory heat that motion of the same 
ethereal medium, which has been shown by Mr. King, (Mor- 
sels of Criticism. 178 6. p. 99,) and M. Pictet, (Essais de Phy- 
sique. 1790,) to be as capable of reflection as light, and by Dr. 
Herschel to be capable of separate refraction. (Phil Trans, for 
1800. p. 284.) How much more readily heat is communicated 
by the free access of colder substances, than either by radiation 
or by transmission through a quiescent medium, has been shown 
by the valuable experiments of Count Rumford. It is easy to 
conceive that some substances, permeable to light, may be unfit 
for the transmission of heat, in the same manner as particular 
substances may transmit some kinds of light, while they are 
opaque with respect to others. 
On the whole it appears, that the few optical phenomena 
which admit of explanation by the corpuscular system, are 
equally consistent with this theory ; that many others, which 
have long been known, but never understood, become by these 
means perfectly intelligible; and that several new facts are 
