164 



SCIENCE 



[N. S. Vol. XXXI. No. 788 



may be, it has been shown by Swinton^ that 

 when similar oxide mantles are heated to 

 incandescence by cathode rays in vacuo, 

 the presence of one per cent, eeria produces 

 only a very small increase in the luminosity 

 of thoria. It is interesting to note that in 

 the gas flame pure ceria gives about the 

 same light as pure thoria, while in the 

 cathode rays of the Crookes tube, with con- 

 ditions under which ceria gives almost no 

 light, pure thoria gives an intense white 

 light. These facts, which are still unex- 

 plained, illustrate how little is understood 

 in this field. 



I will merely refer to the fact that 

 vapors of gasoline, kerosene, alcohol, etc., 

 are also now used in conjunction with the 

 Welsbach mantles. The field of acet5dene 

 I must also omit with a mere reference to 

 the fact that the manufacture of calcium 

 carbide was a chemical discovery, and the 

 action of water upon it, producing the bril- 

 liantly-burning acetylene gas, was another. 



Turning now to electrical methods of 

 generating light, we find the chemist early 

 at work. Sir Humphry Davy and others, 

 at the dawn of the nineteenth century, 

 showed the possibilities which since that 

 time have been developed into our various 

 types of incandescent and arc lamps. We 

 naturally attach Mr. Edison's name to the 

 development of the carbon incandescent 

 lamp, because it was through his inde- 

 fatigable efforts that a practicable lamp 

 and illuminating system were both devel- 

 oped. It had long been known that plat- 

 inum, heated by the current, gave a fair 

 light, but it melted too easily. A truly 

 enormous amount of work was done in at- 

 tempts to raise the melting-point of the 

 platinum, and the effect of occluded gases, 

 of annealing, of crystalline condition, etc., 

 were most carefully studied, but the results 

 were unsatisfactory. He was therefore led 



= Proc. Roy. Soc, 65, 115. 



to the element carbon as the next most 

 promising conductor of high melting-point. 

 Edison's persistent and finally successful 

 attempts to get a dense, strong, practical 

 filament of pure carbon for his lamps is 

 one of the most encouraging lessons to the 

 chemist of to-day. This history needs to 

 be read in the light of the knowledge of 

 carbon at that time and the severe require- 

 ments of a commercially useful carbon 

 filament. It illustrates the value of con- 

 tinued effort when it is based on knowledge 

 or sound reasoning. The search was not 

 the groping in the dark that some of us 

 have imagined, but was a resourceful search 

 for the most satisfactory, among a multi- 

 tude of possible materials. From our point 

 of view, all subsequent changes in choice of 

 material for incandescent lamp filaments 

 have been dictated by the knowledge that 

 high melting-point and low vapor tension 

 were the first requirements. If you will 

 consult the curve of the melting-points of 

 all the elements, as plotted against their 

 atomic iveights, you will see at once that 

 the desired property of high melting-point 

 is a periodic function of the atomic weight. 

 And it is this fact, which was independ^ 

 ently disclosed as a general law by ]\Ieyer 

 and Mendeljeff, in 1869, that has aided in 

 the selection of all the new materials for 

 this use. You will notice that the peaks of 

 the curves are occupied by such elements 

 as carbon, tantalum, tungsten, osmium, 

 etc., which are all lamp materials. 



A study of the laws of radiation also 

 soon played a part in incandescent lamp 

 Avork. The early rough and black filament 

 of bamboo was first replaced by a polished 

 black carbon filament, and later by one 

 which had a bright, silver-gray coat of 

 graphite. A black body at any tempera- 

 ture radiates the maximum possible energy 

 in all wave-lengths. Heated to incandes- 

 cence, it will radiate more invisible and 



