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Q: I have noticed that many people tend to sneeze when they go from
dark conditions into very bright light. What is the reason for this? (Continued)

* * *

A: Here are some early thoughts on the subject of light sneezing from
Francis Bacon’s Sylva Sylvarum (London: John Haviland for William Lee, 1635,
page 170): ‘Looking against the Sunne, doth induce Sneezing. The Cause is,
not the Heating of the Nosthrils; For then the Holding up of the Nostrills
against the Sunne, though one Winke, would doe it; But the Drawing downe
of the Moisture of the Braine. For it will make the Eyes run with Water;
And the Drawing of Moisture to the Eyes, doth draw it to the Nosthrills,
by Motion of Consent; And so followeth Sneezing; As contrariwise, the Tickling
of the Nosthrills within, doth draw the Moisture to the Nosthrills, and
to the Eyes by Consent; For they also will Water. But yet, it hath been
observed, that if one be about to Sneeze, the Rubbing of the Eyes, till
they run with Water, will prevent it. Whereof the Cause is, for that the
Humour, which was descending to the Nosthrills, is diverted to the Eyes.’
(sic)

C. W. Hart Smithsonian Institution, Washington DC

Wear and where?

Q: Much is made of the problem of recycling the millions of used vehicle
tyres. But what has happened to all the rubber which used to be on these
tyres? If it is worn off as cars travel why don’t the roads slowly develop
an increasingly thick layer of tyre rubber? Rubber-coated roads would presumably
result in less tyre wear, but this does not seem to be the answer.

* * *

A: Tyre rubber is deposited on roads, directly and as airborne dust.
It is washed away by rain – hence black water in puddles – and carried
into drains. Soft road surfaces can reduce tyre wear, but they also reduce
tyre grip in wet conditions and increase road wear in dry conditions. Less
inconvenience accrues from sporadic renewal of tyres than from the closure
of a road for resurfacing.

Alan Calverd Bishops Stortford, Hertfordshire

* * *

A: Ordinary road tyres don’t leave large amounts of rubber on the roads
with the exception of skid marks. However, racing car tyres, being untreaded
(or slick) and made of softer rubber compounds do leave large amounts of
rubber on the road, especially at corners. If you look at a racing circuit
it is possible to see the ‘racing line’ taken by the drivers. This is marked
by a darker area of road caused by a build-up of rubber on the surface.
This rubber is also shed from the tyres in small balls and it is pushed
off the racing line by the action of the cars. So the road away from the
racing line often has less grip because of the presence of these balls of
rubber (referred to as ‘marbles’ in TV motor racing coverage).

Guy Dawson No address provided

What’s the rub?

Q: How does a pencil rubber work? Do all types – conventional, plastic
and putty – work in the same way?

* * *

A: A pencil mark actually consists of graphite particles abraded from
the pencil point by the paper. These particles, which have an angular, gritty
look under the microscope are, for an HB lead pencil, typically between
2 and 10 micrometres in diameter. The particles lie slightly below the surface
of the paper, interlocked between its fibres. A single rub using a rubber
sufficiently soft to reach between the fibres will pick up most of them.
Inspection of the rubber shows the undamaged particles adhering to the surface.
An effective erasing material is also abraded by the paper surface, producing
the familiar small spindles of rubber or eraser material, which wrap up
the graphite particles. At 200times magnification, these look like roly-poly
puddings studded with graphite raisins.

John Rowland Derby

Floating on air

Q: Water vapour is 800 times more dense than air, and ozone nearly twice
as dense. Yet water vapour floats in cloud layers and ozone in a layer up
to 50 kilometres high. How?

* * *

A: Water vapour is actually less dense than air. It is liquid water
at sea level that is roughly 800 times more dense. However, liquid water
droplets and solid ice crystals do form clouds but they do not float. Because
the droplets and crystals are so small, and therefore fall very slowly,
they are effectively carried by the wind and, to a person on the ground,
they appear to float.

Ozone too does not float. It forms preferentially at heights around
50 kilometres from chemical dissociation of molecular oxygen by ultraviolet
sunlight and is more easily destroyed at other levels (or in situ by chlorofluorocarbons)
once it is carried there by diffusion and turbulence.

David Pedgley Wallingford, Oxfordshire

* * *

A: The density of an ideal gas at a particular pressure and temperature
is proportional to its molecular mass. Water vapour has a molecular mass
of 18, and is therefore lighter than other major constituents of the atmosphere
– oxygen (32), nitrogen (28), carbon dioxide (44), ozone (48). If, however,
there is enough water vapour, some condenses to form the liquid water droplets
and ice crystals that make up clouds. The droplets are denser than the air
around them and fall under the effect of gravity, but the viscous drag of
the air prevents them reaching any great speed. Their downward velocity
is usually much smaller than the upward velocity of the rising warm, wet
air which created the cloud in the first place, so the cloud does not fall.
If a droplet grows big enough, or enough of them stick together to make
a larger droplet, then gravity will get the better of viscosity and the
drop will fall as rain.

Oxygen and nitrogen make up approximately 21 and 78 per cent of the
atmosphere respectively. The heavier oxygen does not settle out to the
bottom of the atmosphere, because the tendency of the winds and the thermal
motions of the molecules to mix the gases together is much greater than
the tendency of gravity to separate them out. However, this ceases to be
true in the very highest layers of the atmosphere, above 120 kilometres.
Carbon dioxide makes up only 0.035 per cent of the atmosphere and is fairly
well mixed, like oxygen and nitrogen, but its amount is slowly increasing
because of human carbon-burning activities. Ozone is not well-mixed, because
it is generated and destroyed in the stra-tosphere by processes which happen
over a shorter time than it would take to be mixed by atmospheric circulation.

H. C. Pumphrey Department of Meteorology, University of Edinburgh

This week’s questions – Sleeping sickness:

Why is a yawn infectious? Why, as you read this, do you find yourself
beginning to yawn?

Morris Slater Glasgow

This week’s questions – Shiny, happy hair:

Why, if hair is dead, does my physical or emotional state affect its
condition? When I am ill, my hair is dull and lifeless. Conversely, when
I was pregnant, my hair was lustrous and shiny.

Anne Dove Swansea