RELATION OF HEAT AND LIGHT. 189 



red-hot iron cooled, calculating the heat lost by the time of cooling. Mercury 

 boils at the temperature of 662 ; and yet it is certain that it emits no sensible 

 light, since it is perfectly invisible in a dark room. Mr. Daniel, from experi- 

 ments made with his pyrometer, fixed the temperature of incandescence at 

 980 ; but this, again, is proved to be higher than the true temperature of in- 

 candescence, since antimony, at its fusing point, is visible in the dark, and yet 

 this metal melts at 810. Sir Humphrey Davy fixed the temperature of in- 

 candescence at 812. 



The uncertainty attending the temperature at which incandescence commen- 

 ces cannot be surprising when we consider that besides the difficulty of accu- 

 rately measuring high temperatures, there are no other means of determining 

 the fact of incipient incandescence than the evidence of the sight. Now there 

 are many reasons for concluding that sight is a very imperfect measure of illu- 

 mination. Objects illuminated in different degrees, exhibited to the same in- 

 dividual, will give him very imperfect notions of their actual comparative 

 brightness. Let two pieces of white paper be differently illuminated by com- 

 mon candles : let one be exposed to the light of a single candle, and the other 

 to the light of ten candles, and let them be viewed by any number of individu- 

 als ; it will be found that no two will agree in their estimates of the relative 

 degree of illumination. If, then, the eye be so imperfect a judge of the degree 

 of illumination, it is extremely probable that when the illumination becomes so 

 faint as to be barely perceptible, it will begin to be perceived by different per- 

 sons when it arrives at different degrees of intensity. It is extremely proba- 

 ble, if not certain, that the same object placed in a dark room will be pro- 

 nounced to be luminous by one person, and not so by another ; and it is abso- 

 lutely certain that an object may be luminous to the eyes of certain animals 

 when it is perfectly invisible to the human eye. Sight, therefore, is by no 

 means a certain test of the presence of light, and, consequently, is an extremely 

 inadequate means of determining the commencement of incandescence. If, 

 however, incandescence be defined to be the commencement of that state in 

 which, whether light be actually emitted or not, sufficient light is emitted sensibly 

 to affect the human eye, then the temperature of incipient incandescence must 

 be taken as the average or mean of the results given by different observers. In 

 this sense we shall not, perhaps, be very wide of the truth if it be fixed at a 

 temperature of between 700 and 800. To attempt to fix the temperature 

 more accurately would be inconsistent with the results of experience, and the 

 imperfect nature of our means of estimating them. 



Analogy would lead us to conclude that all bodies in the solid and liquid 

 state are susceptible of incandescence. Since analogy, likewise, countenan- 

 ces the supposition that all bodies are susceptible of existing in these states, it 

 is likewise probable that all bodies whatever are susceptible of incandescence. 

 Practically, however, the attainment of the state of incandescence is rendered 

 impossible in a vast number of bodies, from various causes. In some cases, 

 long before the requisite increase of temperature can be attained the forces 

 which hold the constituent parts of bodies together are destroyed by the antag- 

 onist forces introduced by the heat itself; so that the body is decomposed or 

 resolved into its constituent parts. In other cases combustion takes place, 

 by which the body to which heat is communicated, or some parts of it, com- 

 bine with other elements, and form new compounds. These circumstances 

 destroy the identity of the body, and cause a total change in its nature and con- 

 stitution, long before incandescence can be looked for. 



It is generally held that air and the gases form an exception to this general 

 effect. No heat ever yet attained has rendered a body in the gaseous form red- 

 hot ; and yet such bodies have been certainly raised to a temperature sufficient 



