I've seen The Barking Dog probably a
hundred times maybe more and I've never
noticed this before because it's just
too fast for the eye to
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see this reaction is one of our real
favorites it's been carried out for many
years as a demonstration to our students
and particularly by by myself and some
of my colleagues but also by um BD Shaw
Colonel Shaw who used to do the thunder
and lightning lecture many many times
and app PL
light it's called The Barking Dog it
produces quite a kind of a a barking dog
sound a a wolf sound
it involves a test tube a really big
test tube this is just a model with a
cork and inside is a mixture of carbon
disulfide
CS2 and n2o nitrous oxide the the flame
shoots out the top of the test tube so I
guess you have this image of this if
this flame coming up and and consuming
your hand if you leave it over the top
of the test tube too long um and my
first few goals I was quite you you can
see me flinching in some of the videos
but um as I've done it now probably tens
if not hundreds of times I Flinch less
and
less the combustion front races to the
bottom of the tube giving up this
amazing whoop and normally what the
audience sees is a
great the note just goes up like the
squeak I made then
it can be drawn out it could be
wof or sometimes it goes much quicker
but when you watch it in slow motion it
looks quite
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different now you may not have seen
exactly what happened what I observed
was that although the flame started
slowly it built up speed as it went on
I've never seen it using this really
really Ultra fast camera you still see
the flame going down but instead of
going down in one great whoo it jumps up
and down the tube so it goes down a
little bit and then up and down and up
and down and so on and as it gets nearer
the bottom it seems to go up and down
more and more okay so we take the top
off we'll let a
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match and I thought that this this this
flame front used to go very very
smoothly from the top to the bottom and
hence the whoop but actually what this
video footage has shown is that it
actually goes in stages it sort of
goes it sort of chugs its way to the
bottom of the tube we weren't quite
sure what was happening I think what's
happening is the pressure wave is
bouncing off the bottom of the tube now
a number of very curious things are
going on in this tube imagine this hand
here the one with the watch is the flame
front coming down and as it comes down
it gradually goes FAS
as it heats up and burns the fuel and
eventually gets the stage where it
almost explodes and sends a shock wave
down to the bottom of the tube when it
gets the bottom of the tube it can't go
any further it's not strong enough to
break the glass the only time it ever
went wrong for me was we broke the
bottom of the glass so I guess that the
pressure pushed the the tube downwards
and we broke the top off the end and I I
only know of that happening once and it
happened to me so it's reflected and
comes up again and then when it gets the
flame it gives it a push
upwards so the flame is now burning up
here where there isn't quite so much
fuel and it gradually warms up again
till it gets here where the fuel is Rich
and then another shock wave goes boom up
again
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and actually if you look really
carefully it looks like it's picking up
speed as it goes down the tube and I
think that's because the distance
between the the flame and the bottom of
the tube is getting smaller and smaller
each time boom
up till in the end it's going really
quite fast well we still have another Tu
prepared and so you have another
opportunity of observing this effect
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I've seen two or three Barking Dog
experiment since then but the flame
front going down is really too fast to
see this bouncing and anyway I'm too
busy blocking my ears to really pay
attention to the flame front
I think of all the slow motions that
I've watched that one was the most
surprising because you could have
expected that cesium or rubidium would
cause a big explosion but none of us
really expected this motion of the flame
front I suppose once you've seen it you
can think of an explanation it's no
longer such a surprise but none of us
were expecting it okay whole PhD is in
some of these videos every time you do a
new experiment you see something subtly
different and that just asks it makes
you ask more questions
red circle red circle
yep Ready on the trigger
ready yeah