SENSITIVE FLAMES. 
165 
giving the appearance shown in fig. 4. The crossing of the 
images is due to the to-and-fro motion of the mirror. The 
isochronous vibration of the flame that is here revealed, gives 
rise to a musical note in the case of the divergent flame. 
But whilst a sensitive flame is most affected by sounds of a 
definite pitch, it can also respond to a certain range of vibra- 
tions, so that within limits the higher the pitch of the exciting 
sound, the more rapid the vibration of the flame. A fine 
experiment illustrating this fact can be made by sounding a 
musical instrument near the divergent flame, at the same time 
looking at its reflection in a moving mirror. With a compara- 
tively low note, the images of the flame stand widely apart, as 
shown in the plate at fig. 4. Raising the pitch of the note, the 
flame vibrates more rapidly, the images seen in the mirror 
become more numerous, and link themselves into a reticulated 
pattern of exquisite beauty. This is shown in fig. 5, but in 
both cases the representation falls far short of the beauty of the 
real appearance. As the note rises still higher in pitch, the 
network becomes finer and finer, until the eye can no longer 
separate the images in the mirror. The value of a flame in 
searching the condition of a vibrating body can here be seen, 
for in the moving mirror it is easy to detect the frequent ad- 
mixture of higher notes with the fundamental tone. At such 
times small and numerous images of the flame are observed to 
ride on the backs of the large and widely-separated ones. By 
means of a concave mirror moved to-and-fro, this effect can 
easily be thrown on a screen, and many points in acoustics may, 
in this way, be demonstrated in the simplest manner. 
Thus have we sought to translate the phenomena of these 
sensitive flames from the “ disorderly mystery of ignorance ” into 
the “ orderly mystery of science.” For, notwithstanding any 
explanation which may be given, the facts remain as wonderful 
and mysterious as ever. And science does not remove this 
mystery, does not lessen this wonder : it teaches us that we are 
surrounded by wonders, that we are enveloped in mystery ; and, 
at present, it can do but little more than reveal these wonders, 
classify these mysterious facts, and awaken a right and intelligent 
appreciation of them. 
March 1 1th, 1867 . 
P.S. — Whilst these sheets were passing through the press, 
subsequent experiments and further consideration have led me 
to supplement and to simplify the explanation given in this 
article. It has been shown, that a sensitive flame is one which, 
on the slightest mechanical increase in the pressure, or, what 
comes to the same thing, in the velocity of the gas as it issues 
