1908] 
Monazite and Monazitr Mining 
85 
ing has to be burned off. It is estimated that the American mar- 
ket consumes 40,000,000 of these mantles per year. 
Another element obtained from the monazite is didymium, 
whose oxide is dark brown . Use is made of this for branding the 
mantles with an indelible brand. A nitrate solution is made and 
an ordinary rubber stamp used for branding. 
Of the associated minerals, zircon has a commercial value of 20 
to 25 cents per pound for its zirconia content, which is used in 
the manufacture of the glower of the Nernst lamp. The funda- 
mental principle of this Nernst lamp is that certain of the rare 
earths or refractory oxides will conduct an electric current and 
glow after they have been heated to redness. This discovery, 
which was made by Dr. Nernst in 1897, has resulted in the devel- 
opment and perfecting of the glower which is now embodied in the 
Nernst lamp. . This glower is composed of a mixture of the rare 
earth oxides and is made in the form of a small rod or pencil of 
chalk-like material, having wire terminals at either end. When 
cold, the glower is an insulator, but by means of the wire the 
glower becames heated to redness when a current is passed through 
these wires, and its resistance gradually decreases until it has 
reached a red heat, when with 220 volts across the terminals it 
starts to conduct the current and give light. 
In bringing a glower up to its starting point corresponding to a 
temperature of 1,200° F., use is made of a small electrical heater 
composed of two or more small tubes of porcelain, about 1-J- inches 
long and i inch in diameter, which are overwound with fine plati- 
num wire, this in turn being held in place and protected from the 
intense heat later generated by the glower by an outer coating of 
porcelain paste. After the glower becomes heated, there is, of 
course, no further use for the heater, and it is cut out by a small 
electro-magnet cut-out, which consists of a magnetic coil connect- 
ed in series with the glower, an armature, and the necessary con- 
tacts in the heater circuit. Thus, when the glower has become 
heated sufficiently, the current begins to pass through it, and 
when this becomes sufficiently strong the armature is attracted 
and the contacts are separated, thus disconnecting the heater from 
the line. The surface of the glower before being used presents a 
