194 



RELATION OF HEAT AND LIGHT. 



more than the one thousandth part of the effect of the direct unconcentrated 

 light of the sun. Now it was found that, under favorable circumstances, the 

 sunlight, acting on the bulb of a thermometer, caused it to rise about 230 ; 

 it follows, therefore, that the effect of the concentrated light of the moon, in 

 the experiment just mentioned, could not exceed the fifth part of a degree ; , 

 but even this is greater than its true effects, because the light of the moon has / 

 been here compared with the light of a cloudy day, which is less intense than (l 

 the direct rays of the sun. From this and other reasons, it is probable that, 

 admitting the moon's rays to possess the calorific power, they could not, in 

 the experiment of De-la-Hire, affect the thermometer to an extent even of the 

 twentieth of a degree. 



There are certain bodies which, at a comparatively low temperature, possess 

 the property of emitting light, presenting an appearance of a lambent flame, the 

 color being different in different bodies, and apparently depending on the color 

 of the body itself; this process is called phosphorescence. The minerals which 

 possess this property in the highest degree, are fluorspar and phosphate of 

 lime. Some bodies exhibit this effect at the commencement of spontaneous ' 

 combustion. Certain kinds of meat and fish, when putrefaction begins, are ( 

 luminous in the dark. If four drachms of the substance of whiting, herring, or 

 mackerel, be put into a phial containing two ounces of sea-water, or of pure 

 water holding in solution half a drachm of common salt, the phial, when ex- 

 posed in a dark place, after the lapse of three days, exhibits a luminous ring 

 on the surface of the liquid. The whole liquid, when agitated, becomes lu- 

 minous, and continues so for some time. When these liquids are frozen, the 

 phosphorescence disappears, but it reappears when they are again thawed. A 

 moderate increase of temperature causes an increase in the luminous appear- 

 ance, but a boiling heat extinguishes it. The light thus produced has no sen- 

 sible effect on the thermometer. 



