888 INCANDESCENT MANTLES. 



worker whose practical appreciation of the importance of his discovery 

 resulted in one of the most phenomenal successes of modern times. 



The term "rare earths" is one of those anomalies which mar the 

 vaunted precision of science, as although it might justly be applied to 

 tlie oxides of many metals, it was in 1SS5 used to designate a small 

 l)and of metallic oxides which occur in certain rare minerals, of which 

 cerite and gadolinite may be taken as types. 



The term '"earth" was applied to such l)odies as the oxide of chro- 

 mium, C'raO.,; and alumina. Ai,,();,, wiiile the ''alkaline earths" were 

 the calcium and magnesium groups. 



Thes<> rare earths were generally considered to be: (Vrium oxide, 

 CcjO.,; lanthanum oxide, LaoO.,; didymium oxide, D^'^O.,; ytti'ium 

 oxide, YgO^; erbium oxide, KraOj; together with some others even 

 scarcei-, and these are divided i>ito two gi-ou|)s: 



('(•rite onrths. YitiTite earths. 



Cerium (jxide, Yttrium oxide, 



Lantiianum oxide, Krhinin oxide, 

 Didymiiun oxide, 



which are easily divided from each other by the action of an excess of 

 potassic sulphate, the cerite eartiis foiniing "alums" which are insol- 

 uble in potassic suh)hate and so s(>parate out. while the ytterite earths 

 r(>main in solution. These rare earths are all bases, but there are 

 other metallic oxides, ecpially rare, which })lay an important part in 

 mantle making, but can not be classiiied with them. Thoria, ThOg, 

 and zirconia, ZrOa, are not earths, and in some combinations' show 

 rather an acididous than a l)asic tendency. These two metals, thorium 

 and zirconium, are classed with titanium, silicon, etc. 



It was with the study of these oxides that Welsbach was most con- 

 cerned at the period when his early attempts began to assume tangil)le 

 shape, and having discovered the possibility of making a mantle to lit 

 the shape of a nonluminous Bunsen tiame b}'^ soaking a cotton fabric in 

 a solution of salts of the rare earths and then burning out the organic 

 matter in such a way as to leave a ghost of the departed threads built 

 up of the oxides of the metals used — thin enough to be excited to 

 luminosity by the heat of the flame, and yet resistant enough to retain 

 its shape under the temperature to which it was submitted — he took 

 out his celebrated patent of 1885, which has proved an efficient first 

 line of defense against those who, left without the charmed circle, yet 

 have hungered for the baked meats within. 



In the 1885 patent Welsbach protects the idea of making a mantle 

 by saturating the cotton fabric and then burning ofi', using mixtures 

 of the salts which he gives as "60 per cent zircouia or oxide of zir- 

 conium, 20 per cent oxide of lanthanum, 20 per cent oxide of yttrium. 

 The oxide of yttrium may be dispensed with, the composition then 

 being 50 per cent zirconia and 50 per cent oxide of lathanum. Instead 



