CHEMICAL SCIENCE. 243 



Table showing the comparative cost of light from various sources, each equal 

 to 20 sperm candles burning 120 grains per hour each, for 10 hoiirs : 



s. (L 



Wax, 7 2 



Spermaceti, ....... 6 8 



Tallow, 28 



Sperm oil, (Carcei's lamp,) . . . . 1 10 



London gases, . . . . . 04^ 



Manchester gas, ...... 3 



Another London gas, . . . . . 2 



We will, therefore, confine our attention principally to the light pro- 

 duced from vegetable fuel, in considering the economical and sanatory 

 bearings of artificial light. The production of artificial light depends 

 upon the fact that at certain high temperatures all matter becomes lumi- 

 nous. The higher the temperature, the greater the intensity of the light 

 emitted. The heat required to render matter luminous in its three stages 

 of aggregation differs greatly. Thus solids are sometimes luminous at 

 comparatively low temperatures, as phosphorus and phosphoric acids. (A 

 jet of name produced by the fusion of these substances was exhibited, 

 and the temperature shown to be quite inadequate to the ignition, or even, 

 scorching, of the finest cambric or gun-cotton.) Usually, however, solids 

 require a temperature of 600 or 700 F. to render them luminous in the 

 dark, and must be heated to 1000 F. before their luminosity becomes 

 visible in daylight. Liquids require about the same temperature. But 

 to render gases luminous, they must be exposed to an immensely higher 

 temperature ; even the intense heat generated by the oxyhydrogen blow- 

 pipe scarcely suffices to render the aqueous vapor produced visibly lumi- 

 nous ; although solids, such as lime, emit light of the most dazzling splendor 

 when they are heated in this flame. Hence, those gases and vapors can 

 only illuminate which produce or deposit solid or liquid matter during 

 their combustion. This dependence of light upon the production of solid 

 matter is strikingly seen in the case of phosphorus, which, when burned in 

 chlorine, produces a light scarcely visible, but, when consumed in air or 

 oxygen, emits light of intense brilliancy ; in the former case the vapor of 

 chloride of phosphorus is produced, in the latter solid phosphoric acid. 

 Several gases and vapors possess this property of depositing solid matter 

 during combustion, but a few of the combinations of carbon and hydrogen 

 are the only ones capable of practical application ; these latter compounds 

 evofVe during combustion only the same products as those generated in 

 the respiratory process of animals, viz., carbonic acid and water. The solid 

 particles of carbon which they deposit in the interior of the flame, and 

 which are the source of light, are entirely consumed on arriving at its 

 outer boundary ; their use as sources of artificial light under proper regu- 

 lations is, therefore, quite compatible with the most stringent sanatory 



