INCANDESCENT MANTLES. 389 



of using the oxide of yttrium, ytterite earth, and instead of the oxide 

 of lanthanum, cerite earth, containing- no didymium and but little 

 cerium, may be employed." v 



As a commercial article, the mantles made under this patent were 

 dire failures; they gave a candlepower which varied from H to 6 can- 

 dles per foot of gas consumed, and were so friable that the mantles of 

 to-day seem giants of strength as compared with their puling fore- 

 fathers. They, however, fullilled the important function of launching 

 the idea of the mantle, and hardly had the patents been taken out than 

 Welsbach made the further discovery that by going outside the group 

 of rare earths, and by replacing zirconia as a basis by thoria, increased 

 life and strength could be given to the mantle, and this was protected 

 by the patent taken out by Welsbach in 1886. 



In this patent he protects the use of thoria alone or in admixture 

 with zirconia, lanthana, yttria, didymia, erbia, magnesia, or alumina. 

 During the next few years the composition of the mantles made by 

 the Welsbach companies was of a ver}^ varial)le character, but the 

 largest proportion of them consisted of commei'cial thoria and gave a 

 light of 6 to 8 candles per foot of gas, this being due to the fact that 

 the material used was not pure and contained traces of ceria, which 

 gave it the power of emitting what was then a considerable amount of 

 light. 



Pure thoria gives a practically nonluminous mantle, and with chem- 

 ically pure material the light obtainable is under 1 candle per cubic 

 foot of gas, while even now a mantle made from the commercially 

 pure thoria, manufactured for mantle making, gives a light of from 4 

 to 5 candles per cubic foot of gas, this being entirely due to the diffi- 

 culty of separating the last traces of ceria; and as 0.1 per cent of ceria 

 causes the thoria to give about 5 candles per foot of gas, it is difficult 

 to obtain a sample which does not give a certain amount of light. 



The use of ceria together with thoria is mentioned in some of Wels- 

 bach\s early foreign patents, but the exact date at which it was first 

 realized that ceria in traces had the marvelous effect on the light 

 emissivity of the thoria mantle that we find in the mantles of to-day is 

 not very clear, but it is evident that the advantage of the presence of 

 small (juantities of ceria was beginning to be realized by 1891, when 

 Mr. W. McKean, the chemist of the English Welsbach Company, read 

 a very interesting paper before the Society of Chemical Industry and 

 pointed out that ceria is by no means a disadvantage in small quanti- 

 ties, as it adds to the constancy of the illuminating power, and he 

 gives a table showing the influence of the presence of ceria in the 

 lighting fluid, and also the influence upon the light which increasing 

 percentages of ceria have. Thus, for instance, the ordinary lighting 

 fluid contains 0.25 per cent of ceria, gives 25 candles for a consumption 

 of 2.5 cubic feet of gas, or 10 candles per foot, while increasing the 



