549 
which, when expanding, rang a signal bell. This form of 
apparatus was quite successful. I shall take the description 
of the apparatus now used from an article in the Chemical 
News, of January 11th, 1867, because haying Mr. Ansell's 
expression of satisfaction with the accuracy of that account. 
" Mr. Ansell proposes to use an instrument of which fig. 1 is 
a sketch. It consists of an iron funnel provided with an iron 
Fig. l. 
TJ-tube, the end of which is closed by a piece of glass tube 
fixed in brass, to which one pole of a battery is attached ; the 
upper part of this glass tube carries a brass collar, through 
which passes an adjusting screw, to the lower end of which 
is fastened a piece of copper wire with a platinum point. 
Mercury is poured into the iron funnel till it rises in the 
glass tube to a convenient height. This mercury is allowed 
to find its level by the opening of a valve, when setting the 
instrument. The septum is a tile of Wedgewood's ware, 
and closes the open part of the funnel, good sealing wax 
being the best cement for securing it in its place. The other 
battery wire is connected with the instrument, so that, if 
diffusion take place, the mercury is pressed up against the 
platinum point, and thus communication is established. Mr. 
Ansell has found that this instrument gives warning in four 
seconds if the mixture of gas be still below the point of 
explosion ; but, by adjusting the point, so that there is not 
more than the thickness of a shilling between it and the 
