Erbium

Erbium (sym. Er, eq. 170·5) is a rare metal, the compounds of which are present in the mineral gadolinite, found at Ytterby in Sweden. It is also believed to be present in the sun’s vapour. It is found along with compounds of yttrium and, it is alleged, of terbium, also rare metals, the existence of one of which, terbium, is still questioned. It is chiefly of interest because the compounds of the three metals mentioned resemble each other so closely, and have such similar properties, that the processes of separation and purification are difficult and tedious in the extreme, a single research with the object of obtaining absolutely pure salts requiring years of patient work.