208 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
A piece of millboard or of glass, a plank of wood, or the hand placed across the open 
end Y of the tunnel X Y (Plate XVIII.) intercepts the sound of the bell B and stills 
the sensitive flame F. 
An ordinary cambric pocket-handkerchief stretched across the tunnel end produced 
hardly an appreciable effect upon the sound, the flame being sensibly as much agitated 
when it was present as when it was absent. Sending the sound through two layers of 
the handkerchief, the flame continued to be much agitated; through four layers the 
flame was still agitated, while through six layers the flame, though nearly stilled, was 
not entirely so. 
Dipping the same handkerchief in water, and stretching a single wetted layer across 
the tunnel, it stilled the flame as effectually as the millboard or the plank of wood. It 
is obvious therefore that the sound-waves in the first instance had passed through the 
interstices of the cambric. 
Through a single layer of a thin silk handkerchief the sound passed without sensible 
interruption; through six layers of the same handkerchief the flame was strongly 
agitated ; while through twelve layers the agitation was quite perceptible. Looking at 
the sun, a feeble luminosity was perceived through six layers of the silk, while twelve 
layers totally intercepted the light. It would be easy to multiply instances such as this 
of bodies opaque to light and transparent to sound. 
A single layer of this silk, when wetted, stilled the flame. 
A layer of soft lint produced but little effect upon the sound ; a layer of thick flannel 
was almost equally ineffectual. Through four layers of the flannel the flame was per- 
ceptibly agitated by the sound. Through a single layer of green baize the sound passed 
almost as freely as through air ; through four layers of the baize the action was still 
sensible. Through a layer of close hard felt, half an inch thick, the sound-waves 
passed with sufficient energy to sensibly agitate the flame. 
Oiled silk has no sensible interstices ; a single thin layer of this substance stopped 
the sound and stilled the flame. A single layer of goldbeater’s skin did the same. A 
leaf of common note-paper, or even of foreign post, stopped the sound. A single column 
of heated air rising from a Bunsen’s flame in front of the tunnel has been proved by 
Mr. Cottrell sufficient to still the sensitive flame. 
The sensitive flame is not absolutely necessary for these experiments. Let a ticking 
watch be hung 6 inches from the ear, a cambric handkerchief dropt between it and the 
ear scarcely sensibly affects the ticking, a sheet of oil-skin or a heated gas column cuts 
it almost wholly off. 
But though oiled silk, foreign post, and even goldbeater’s skin can stop the sound, 
when the film becomes sufficiently thin to yield freely to the aerial pulses the sound 
is transmitted through the film. A thick soap-film produces a sensible effect upon the 
flame, a very thin one does not. The augmentation of the transmitted sound may be 
observed simultaneously with the generation and brightening of the colours of the film. 
A thin collodion-film acts in the same way. 
