278 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



According to this table, the Welsbach light is cheapest; next comes 

 the arc light; then follow in order the light of a petroleum lamp, arc 

 light with ground-glass globe, ordinary gas light as furnished by an 

 Argand burner, acetylene light, incandescent electric light, and finally 

 gas light as obtained from the fish-tail burners, which are in most com- 

 mon use. A candle, as you perhaps all know, is even at the present 

 time an article of luxury. 



It must be admitted, that this arrangement is quite arbitrary, as it is . 

 based on the average market price of materials, etc. For example, 

 electric light is decidedly cheaper when supplied by a private installa- 

 tion and if the machines are always loaded to their full capacity. A 

 plant of at least 100 horsepower can supply power at the price of 2J 

 to 3 cents per kilo-watt-hour; for smaller plants this becomes 5 or 6 

 cents. In the first case the arc light would be the cheapest, and the 

 electric incandescent light would come third. 



This order of value is entirely upset when we consider other condi- 

 tions besides the question of cheapness as influencing the value. 

 According to the purposes which a light is to serve these conditions 

 are very different. For example, the very cheap arc light can not for 

 one moment be considered for illuminating dwelling rooms, for it is not 

 sufficiently divisible, and similarly the relatively more expensive incan- 

 descent electric light far surpasses the incandescent gas light on account 

 of its enormous sanitary advantages. 



Part II. 



THE NATURE OF THE DIFFERENT KINDS OF LIGHT. 



The consideration of these secondary actions of a gas flame intro- 

 duces us to the second part of our subject, which deals with the nature 

 of the different kinds of light and with the physical processes involved. 

 In the third part I will try to elucidate the physical laws connecting 

 the luminous intensity of a body with its temperature and with its 

 nature, on which factors the intensity of a source of light depends. 

 We will divide sources of illumination into two classes, namely, those 

 in which the luminosity is produced by highly heating a body and those 

 which produce the sensation of light at a relatively low temperature — 

 cold flames, fluorescence, and others of that character. 



In the first class we must distinguish between free-burning flames, 

 such as a candle, a petroleum lamp, a gas flame, and an acetylene lamp, 

 and electric lights — that is, according to the manner in which the 

 luminous bodies are brought to incandesence. 



Although my lecture Avill deal principally with the theory of lumi- 

 nosity produced by high temperatures, since this alone is the basis of 

 all the sources of light in common use, I can not omit reference to the 

 "cold flames" as long as they point out the direction in which we must 

 look for the light of the future. 



