SENSITIVE FLAMES. 
161 
sensitive flame is obtained of a new character, and of extraordi- 
nary susceptibility to certain sounds.* 
Uninfluenced by sound this flame can be made to attain a 
height of full twenty inches, burning with greater steadiness 
and less smoke than that shown in Plate VIII. fig. 1, but having 
the same long and tapering character as the flame there drawn. 
The effect of sound on this flame reduces it to a very different 
shape from, that of fig. 2 on the plate ; at the noise of a tap it 
shrinks down half its height, or nearly a foot, the upper part 
becomes thick and bushy, the lower remains unchanged ; and, 
instead of burning more steadily and with a better light under 
the influence of sound, it becomes tumultuous and less luminous. 
In this state it is shown at fig. 3 on the plate, having, it will be 
noticed, irregular little coruscations clinging and flickering up 
its sides. We will call fig. 2 the divergent, and this, fig. 3, the 
brush flame. It is a wonderfully delicate flame. Like a sensi- 
tive, nervous person, it starts and shrinks at every little noise; 
the change in its aspect being so great as to cause it to be one 
of the most striking and effective of all the sensitive flames. 
But, most curious of all, this flame is especially sensitive to the 
letter S, or any sibilant sound. Tell it to appear as a “ brush,” 
and it instantly obeys, vigorously assuming its characteristic 
shape, shown in fig. 3. Say to it “hush,” and at once, with a 
spiteful, fluttering noise, it cowers down nine inches. A whis- 
per, too, being rich in sibilants, is at once detected by the flame, 
which crouches, as if to listen, the moment you move your lips. 
Like most sensitive people, it keenly feels the effect of a “ hiss.” 
Let it hear you make such a sound, and it drops from its lofty 
height, shrinking into a foolish, fussy little flame, which, as 
though in agony, fairly begins to roar if you venture to sustain 
that disparaging sound. Even if you go far away and out 
of sight, the flame will shiver all over every time you utter the 
obnoxious sibilant.f 
But this flame, like the others, is not at all sensitive to some 
* This flame was first noticed by Dr. Tyndall, and the associated experi- 
ments shown by him at the Friday evening lecture to which reference has 
been made. To obtain the flame it is not at all essential that the burner 
should be of steatite, but it will be noticed that the orifice is smaller than 
that of the burner previously described. 
f In a note attached to a paper on sensitive flames, published in the 
March number of the Philosophical Magazine , I have drawn attention to 
other remarkable peculiarities which are associated with the sound of the 
letter S. I have frequently noticed that when any sibilant is uttered 
simultaneously by a large assemblage of people, the resulting sound is far 
different in intensity to that of any other — a striking noise being produced, 
like the escape of steam from a high-pressure boiler. It is worthy of 
remark that the Astronomer Royal believes the sound of the letters S or Z 
to be produced in a manner different from the rest; namely, by an in- 
terruption in the continuity of the particles of air. 
