170 



SCIENCE 



[X. S. Vol. XXXI. No. 788 



wards the left, or towards the visible wave- 

 lengths. 



It is this rapid shifting of the position 

 of masimiim energy which makes the search 

 for substances which can withstand even 

 only slightly higher temperatures of such 

 great interest. 



The ci;rves for the black body and for 

 platinum (dotted lines) ai'e not greatly 

 different in genei'al appearance, but the 

 total amount of energy emitted at a given 

 temperature from the black body is shown 

 to be more than for the platinimi, and it 

 can be seen that at about the same tem- 

 perature the platinum is the more econom- 

 ical light source. Professor Lummer has 

 said that at red heat, bright platinum does 

 not radiate one tenth the total energy 

 M'hich the ideal black body radiates at the 

 same temperature, aud at the highest tem- 

 perature still less than one half. The 

 deviation of platinum from the black body 

 law is a step in the direction of getting 

 improved light-efficiency without corre- 

 sponding increase of temperature. This 

 method is practically withoiit limit in its 

 extension, for there seems to be no limit to 

 the forms of energy curves which different 

 substances may possess. The curves are 

 apparently determined not only by phys- 

 ical state, but also by chemical composition 

 of the emitting substance. 



You see before you a vacuum incandes- 

 cent lamp which contains a ribbon of plati- 

 num in the shape of a loop. "Wliile the sec- 

 tion of the platinum is the same through- 

 out, one half of the loop is blackened by 

 depositing a little platinum black upon it. 

 This greatly affects the light efficiency as 

 shown. The blackened portion, being more 

 nearly a black body, radiates at each tem- 

 perature relatively more energy of long 

 wave-length (t. e., heat) than the bright 

 portion. So for about equal total energy 

 radiated the ribbon radiates less as light 

 from the blackened surface. 



In the production of artificial light, the 

 tendency will always be in the direction 

 of increasing the practical efficiency, i. e., 

 reducing the cost of light. We have seen 

 that there is still much room for this. In 

 the case of the kerosene oil lamp we know 

 that much less than one per cent, of the 

 energy of combustion of the oil is radiated 

 as light from the iiame. In the case of the 

 most efficient source — the electric incandes- 

 cent lamp at highest efficiency— we are still 

 far from ideal efficiency. A still higher 

 temperature would yield a yet higher effi- 

 ciency. We do not know exactly how much 

 light might possibly be yielded for a given 

 consumption of energy, but one experi- 

 menter concludes that it is about ten 

 candles per watt. If this is true, even the 

 most efficient light you have seen this eve- 

 ning is less than half as efficient as it might 

 be. Fortunately, it is not now clear just 

 how the chemist is to realize all the ad- 

 vances which he will make in more efficient 

 lights. 



No consideration of this part of the sub- 

 ject is complete without a brief reference 

 to the efficiency of the fire-fly. The source 

 of his illumination is evidently chemical. 

 This much is known about the process: 



The light-giving reaction is made to 

 cease by the removal of the air, and to in- 

 crease in intensity by presence of pure 

 oxygen. It is extinguished in irrespirable 

 gases, but persists in air some time after 

 the death of the insect. Its production is 

 accompanied by the formation of carbon 

 dioxide. These all indicate a chemical 

 combustion process. Professor Langley 

 has shown that such a flame as the candle 

 produces several hundred times as much 

 useless heat as the total radiation of the 

 fire-fly for eqiaal luminosity. In other 

 words, the fire-fly is the most efficient light 

 source known. This is illustrated by the 

 energy distribution curves fi-om several 



