MERCURY AND MERCURY POISONING - THE CONTROVERSIAL ELEMENT (THE APPLICATIONS AND DANGERS OF MERCURY)

Virtually all metals exist as solids at room temperature. Mercury is the only metallic element that is a liquid under normal conditions. If cooled to 39°C, it does freeze to a solid. Liquid mercury is shiny and metalliclooking.

You have probably heard a fair amount about the toxicity of mercury. As a liquid, it is not especially toxic when swallowed since most of it passes through the body unchanged.

However, mercury vapor is highly toxic, as are all compounds of mercury that dissolve in water to form solutions. Once they enter the body, these forms of mercury can attack the brain and produce mental and physiological disturbances.

An incident in Texarkana, on the Texas–Arkansas border, illustrated the hazards of handling mercury. Two teenagers stole 40 pounds of liquid mercury from a site where it had been used to make neon lights.

They poured it over themselves and on floors in their homes, gave it out to friends, and even dipped cigarettes into the liquid and smoked them. Within days they began to exhibit the signs of mercury poisoning: coughing up blood, vomiting, breathing difficulties, and seizures.

The end result was that eight contaminated homes were evacuated, a family dog was killed by the vapors, and more than 170 people in the town and surrounding areas received medical treatment for mercury exposure.

Mercury poisoning was much more common in the 19th century when workers who used mercury to cure felt hats developed twitches, spoke incoherently, and drooled as a result of long-term exposure to mercury vapors.

These workers provided Lewis Carroll with a model for the Mad Hatter in Alice in Wonderland. These days, most of the mercury that enters the environment comes from the incineration of waste and sewage sludge, and the burning of coal.

Recently, scrapping cars without removing the elemental mercury used in light switches and other components was identified as a significant source of the element to the environment.

MERCURY DANGERS IN DENTAL CARE?
Until quite recently, an alloy that most people had an intimate acquaintance with was the material used to fill cavities in decayed teeth. You may be surprised to learn that mercury was one of the metals used to fill teeth.

Although mercury is a liquid at room and body temperatures, it forms many alloys, called amalgams, that are solid at normal temperatures. Those having melting points in the 60°C range are useful for fillings, since they can be placed in the decay cavity as a warm liquid metal without causing the patient pain.

The liquid assumes the cavity shape as it cools and solidifies in place. Dental amalgam combines mercury with silver, which imparts resistance to tarnishing and mechanical strength, and about half as much tin, which readily amalgamates with mercury.

When first placed in a tooth, and whenever the filling is involved in the chewing of food, a tiny amount of the mercury is vaporized. Some scientists believe that mercury exposure from this source causes long-term health problems in some individuals, but an expert panel of the U.S. National Institute of Health concluded that dental amalgams do not pose a health risk.

A recent study of adults found that no measure of exposure to mercury—whether the level of the element in the urine or the number of dental fillings—correlated with any measure of mental functioning or fine motor control.

THE SCIENTIFIC MODEL OF GAS - BASIC TUTORIALS

Perhaps you have had the unpleasant experience of walking down the street and being accosted by the noxious smell of rotten eggs emanating from a nearby sewer. The odor, which is produced by the gaseous compound hydrogen sulfide, will reach your nose even if there is no wind.

This experience confirms the existence of gases in air, and the theory that the particles in a gas are constantly in motion. If they were not, odors would not carry unless there was a wind current present. As the Roman poet Lucretius said 2000 years ago in his epic poem The Nature of Things:

We can perceive the various scents of things

Yet never see them coming to our nostrils

The scientific model for gases is that of independent, tiny particles traveling rapidly in straight-line motion through empty space, as a rocket ship travels through outer space. Owing to the rapid motion of its constituent particles, a gas quickly expands to fill completely whatever space is accessible to it.

As a given gas particle travels through space, it occasionally collides with other gas particles or with the walls of its container if it is in one. These collisions result in a change in direction for the particles—much as a billiard ball changes direction when it hits another ball or hits the side of the pool table.

One of the many pieces of evidence that led to the scientific model for gases is that gases are much easier to compress to a smaller volume than are liquids or solids. Compressing a gas corresponds only to reducing the amount of empty space that lies between the independent particles.

A piece of evidence that led to the notion that the particles in a gas are in constant motion is the fact that a gas exerts a force on the walls of whatever container it occupies. Technically, the pressure exerted by a gas is the amount of force that it exerts on a specified area of surface, say one square centimeter (see Figure a).

For example, the helium gas atoms in a helium-filled balloon are in constant motion. As a consequence of their movements, they often collide with the inside skin of the balloon (see Figure b).

The pressure exerted on the balloon walls by this constant bombardment is sufficient to keep the balloon “blown up,” even though the stretched elastic of the balloon’s material is trying to contract and thereby collapse the interior. Indeed, the balloon collapses only when some of the helium leaks into the air outside the balloon.