164 ARTIFICIAL ILLUMINATION, _ 
steeped in a solution of nitrates of thoria and ceria. It is then dried and 
the whole of the cotton is burnt out by a powerful Bunsen flame, leaving 
behind only a very delicate skeleton composed of thoria and ceria in 
the place of the cotton threads, which, after further heating, or 
“seasoning” as it is called, and a dipping in collodion in order to 
stiffen it temporarily for transit purposes, is the commercial mantle as 
now issued. This collodion is first burnt off when the mantle is mounted 
for use, and then the heat of the Bunsen flame, over which the mantle 
is suspended, raises the delicate filaments of infusible oxides to the 
white and brilliant incandescence which you see now before you. The 
exact composition of the Welsbach mantle is 98 per cent. of thoria and 
2 per cent. of ceria. Thoria and ceria are two of a series of rare 
metallic oxides which includes also zirconia, yttria, lanthana, alumina, 
etc., all of which have been tried for incandescent mantles. The supply 
of thoria, which is the chief constituent of the Welsbach mantle, is an 
exceedingly limited one. Itis derived chiefly from a complex substance 
called monazite, which can be found in North Carolina and a few other 
isolated places. 
The function of the mantle, of course, is to absorb the ‘verse 
energy of the Bunsen flame and to transform as much of it as possible 
into radiant light. We may judge of the efficiency with which it does 
this as compared with the corresponding efficiency of an ordinary gas 
flame from the experiment to which I now call your attention. I have 
here a small No. 2 Bray burner and a Welsbach burner arranged to 
burn alternately off this experimental meter, which shows you that 
they are consuming the same amount of gas, namely 3°8 cubic feet per 
hour. Unfortunately my gas pressure is so deficient, as I have told 
you, that you do not see what you ought to see, but still it is evident 
that the light given out by the Welsbach burner is considerably greater 
than that from the Bray burner. With a proper gas supply it would 
be easy for you, by your personal observation, to approximately 
verify my statement that the light given by the incandescent mantle — 
is eight or ten times as great as that given by such a common burner : 
although the cost per hour is the same for each. 
Of course the objection to the Welsbach system is the extreme 
delicacy of the mantle and its liability to accident. With moderately 
careful management I do not find that this is a serious consideration. - 
Let me give you two or three practical “tips” derived from my own eight - 
years’ household experience of this light. (1) Do not allow it to have too 
much gas: if you do, probably the gas will not be all consumed and — 
some carbon may be deposited on the top of the mantle. After lighting 
the burner you should turn the gas slowly off until the brightest illu- 
mination is obtained. With more gas on it will nearly always be found 
that less lightis given. (2) Should the mantle be blackened by carbon at 
the top, due to the gas having been too full on, the deposit can be easily 
got rid of by simply turning the light down till the mantle is only 
about one-third filled with gas flame. The carbon will then be at the 
top of the flame and will be gradually oxidised or burnt away. (8) 
Should the mantle, from no apparent cause, fail to give approximately 
its original amount of light, it is probable that dust is covering the 
small holes through which the gas emerges in the burner. If the. 
