290 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



We now know, thanks to the researches of Rumford, Joule, Mayer, 

 and Hehnholtz, that heat and molecular motion are identical, and that 

 the greater this molecular motion the greater is the temperature of the 

 body. By hammering, drilling, etc., the materials operated on finally 

 become so hot that they can not be touched without burning the 

 fingers. 



The work performed by our muscles in hammering a piece of lead is 

 partly transformed into heat and thus applied to increasing its tem- 

 perature. The motion of the hammer has been transformed into the 

 motion of the molecules of the lead. This transformation of work 

 into heat follows definite laws, so that a definite quantity of heat is 

 equivalent to a definite amount of work done. The law of the " mechan- 

 ical equivalent of heat" states the numerical relation of one to the 

 other. In nature, where a certain quantity of heat appears to have 

 been lost in the transformation — as, for example, in the production of 

 electric light — it certainly reappears somewhere in the process as heat; 

 that is, as an increase in temperature. The law of the conservation of 

 energy expresses the recognition of this fact. 



Every body in nature must then be considered as consisting of mole- 

 cules in a state of rapid motion. Solids, liquids, and gases only differ 

 in the nature of the motion. If a solid is heated to a sufficiently high 

 temperature it is finally transformed into the liquid or gaseous state. 

 Even carbon evaporates at 3,600° C. Corresponding to this increase 

 in temperature, the motion of the molecules becomes more and more 

 rapid and the energy of the ether waves emitted increases with it. All 

 apparatus which can absorb heat — our skin, a thermopile, a bolometer, 

 etc. — indicate how rapidly the radiation of a body increases with 

 increasing temperature. By means of our differential thermometer we 

 can show the increase of the radiation of the electrically heated plati- 

 num strip with its temperature. The law involving this relation for 

 solids was first stated by Stefan, and was deduced by theory for a 

 so-called "black" body by Boltzmann. If we call the total radiation 

 of the body the sum total of the energy corresponding to all the ether 

 waves emitted, and if we call the absolute temperature of a body its 

 temperature on the Centigrade scale, increased by 273°, Stefan's law 

 of radiation may be stated as follows: The total radiation is propor- 

 tional to the fourth power of the absolute temperature. 



An example will make this law plainer. Let a body at 27° C. be 

 heated to a temperature of 327° C, i. e., from an absolute temperature 

 of 273° + 27° = 300° to one of 273° + 327° = 600°, thus doubling the 

 absolute temperature of the body; but if the absolute temperature is 

 doubled the total energy emitted is increased, according to Stefan's 

 law, 2* or 16 times. If the absolute temperature is trebled the total 

 radiation is increased 3* or 81 times, etc. You will thus recognize how 

 rapidly the energy of radiation increases with increasing temperature. 

 I must not omit to state that the energy maximum of the whole spec- 



