杏吧原创

Dead weight

Question: My school class wants to know why corpses sunk in water eventually
float to the surface.

Answer: Part of my work involves recovering bodies from the River Clyde and
surrounding waterways. Over the years, I have recovered around a thousand bodies
that have spent between a few days and two years in the water.

When a person drowns, water takes the place of air in the lungs. The body
becomes heavier and sinks. It stays there until enough gas builds up inside the
body from bacterial decomposition to make it buoyant and free it from the
suction that silt and mud creates on the river bed, so that it surfaces.

How long this takes depends on the depth and temperature of the water, the
amount of sunlight the corpse receives, and whether it is lying under a ledge or
a bridge. Whether the body is lying on the north or south side of a waterway can
also affect the time it takes to rise, because heating of the water varies on
each bank.

Obviously, the body can take longer to rise if it is underneath something
like a tree, or is caught by an underwater obstruction. I once studied a body
that had risen to the surface with part of a brick wall tied to it by ropes
around the chest鈥攖here were five bricks covered with mortar on each side
of the body and still it floated. What鈥檚 more, if a body is trapped by, say, its
leg, the gases will keep forming until it is buoyant enough to detach itself
from the trapped leg and rise up.

Bodies can also be washed into the side of a river and remain undiscovered,
below an overhanging tree, for example. Debris piles up on top of the body and
silt accumulates. If the body lies out of the Sun鈥檚 rays, which cause much more
rapid decomposition, it can remain there until a flood washes away the silt and
debris.

Of course, if the water is cold enough, the dead body may be held in 鈥渄eep
freeze鈥� and it might never rise.

George Parsonage

Glasgow Humane Society

Answer: Dead bodies undergo a series of changes, biological, chemical and
physical, which eventually return the body鈥檚 components to the food chain. These
changes and the speed of decomposition depend on where the body is.
Decomposition in air is roughly twice as fast as it is underwater and about four
times as fast as underground.

A living person has almost the same density as water: when you breath in, you
float, and when you breath out, you sink. This is why dead bodies sink
initially.

Then the changes begin, and one of the most important is the bacterial
decomposition of sugars and proteins in the tissues鈥攎ainly coliforms,
Clostridium, Pseudomonas and Proteus species. These are
anaerobic bacteria that normally live inside the large intestine, though they
may come from an infected wound, such as gangrene. As they feed on the decaying
flesh, the bacteria excrete gases鈥攃arbon dioxide and sulphur
dioxide鈥攚hich inflate some body parts, mainly the face, abdomen and male
genitals.

Bodies usually stay underwater for between one and two weeks, although this
varies widely and some bodies never come up at all. One of the factors which
affects this is the temperature of the water.

Contrary to common belief, there is no significant difference between the
time it takes for male and female corpses to resurface. There is one notable
difference though: while most corpses rise with spine uppermost, women and obese
persons may rise face-up. This is caused by gas forming in breasts and large
abdomens.

Jan Strojil

Olomouc, Czech Republic

Drip reduction

Question: Why does a water tap, when adjusted to a trickle, reduce to a drip
after a couple of minutes? I have noticed a gas flame also becomes smaller
during this period.

Answer: Flow through a partly open tap reduces because the washer, which was
compressed while the tap was closed, re-expands.

When you want to stop a flow, you have to close the channel completely. It is
very difficult to do this by bringing two hard surfaces together. Perfectly flat
surfaces are expensive and difficult to produce, and if there is any debris in
the water such as loose scale from the pipes, it will spoil the seal that forms
between them. Placing a rubber washer between the faces avoids these problems,
as it doesn鈥檛 need to be completely smooth. The washer is squeezed between the
faces and forms a seal around any particles and irregularities, completely
shutting off the flow.

If you partially open the tap, the initial flow of water diminishes as the
washer re-expands to its natural size鈥攖he size it was before being
compressed. Perfectly explicable, but still an irritation.

Michael Bell

Hitchin, Hertfordshire

Answer: In the case of gas, taps normally seal by metal-to-metal contact,
often in the form of a conical plug, so the effect cannot be explained by a
sealing washer expanding.

A gradually reducing flow of gas may be seen when the pipe between the gas
tap and the gas jet is long or wide. If the tap is wide open and is turned to a
lower setting, there will still be pressure in the pipe between the tap and the
jet. This means the flame will shrink as the flow settles down to the new
pressure.

Kenneth Cragg

Plymouth, Devon

This week鈥檚 questions

Magnificent man: I read in the newspapers the story of a Californian man who
attached a number of weather balloons to his lawn chair and didn鈥檛 come down for
quite some time. How much helium or hydrogen would you need to lift an average
adult off the ground? And how big a balloon would you need to reduce your
apparent weight to one-sixth, as though you were walking on the Moon?

Derek Richardson

Musselburgh, East Lothian

Water weight: I have seen a catalogue from a fire extinguisher manufacturer
that advertised extinguishers filled with 鈥渓ight water鈥�.

What on Earth is light water and, more importantly, why would you put it in a
fire extinguisher?

John Humbach

Etchinghill, Kent