356 On the Measwrement of the [July, 
cumstances thus tend to affect the illuminating power of a standard 
sperm candle. These difficulties, however, are small, compared 
with those which have resulted from the substitution of paraffin, 
&c., for part of the sperm; and Mr. Sugg points out that candles 
can be made with such combinations of stearin, wax, or sperm, and 
paraffin, as to possess all the characteristics of sperm candles, and 
yet be superior to them in illuminating power ; while, on the other 
hand, candles made from the same materials otherwise combined 
are inferior. When, in addition to this, it is found that candles 
containing paraffin require wicks more tightly plaited and with 
fewer strands than those suitable for the true sperm candle, our 
readers will be enabled to judge of the almost unsurmountable diffi- 
culties which beset the present system of photometry. 
But assuming that the true parliamentary sperm candle is 
obtained, made from the proper materials and burning at the speci- 
fied rate, its illuminating power will be found to vary with the tem- 
perature of the place where it has been kept, the time which has 
elapsed since it was made, and the temperature of the room wherein 
the experiment is tried. 
The Rev. W. R. Bowditch, in his work on The Analysis, Puri- 
fication, &e., of Coal-gas, enters at some length into the question of 
test-candles, and emphatically condemns them as light-measurers ; 
one experiment quoted by this author showed that the same gas 
was reported to be 14°63 or 17°36 candle gas, according to the way 
the experiment was conducted. 
The present writer has taken some pains to devise a source 
of light which should be at the same time fairly uniform in its 
results, would not vary by keeping, and would be capable of accu- 
rate imitation at any time and in any part of the world by mere 
description. The absence of these conditions seems to be one of the 
greatest objections to the sperm candle. It would be impossible for 
an observer on the continent, ten or twenty years hence, from a 
written description of the sperm candle now employed, to make a 
standard which would bring his photometric results into relation 
with those obtained here. Without presuming to say positively 
that he has satisfactorily solved all difficulties, the writer believes that 
he has advanced some distance in the right direction, and pointed 
out the road for further improvement. 
Before deciding upon a standard light, experiments were made 
to ascertain whether the electric current could be made available. 
Through a coil of platinum wire, so as to render it brightly in- 
candescent, a powerful galvanic current was passed ; and its strength 
was kept as constant as possible by a thick wire galvanometer and 
rheostat. To prevent the cooling action of air-currents, the in- 
candescent coil was surrounded with glass; and it was hoped that 
by employing the same kind of battery and by varying the resist- 
