# Man's physical universe

THE PROPERTIES OF LIQUIDS 217

The sink-and-float process for

the removal of unwanted materials

from minerals and from coal depends upon the use of liquids of such

a density that one ingredient in a mixture will sink and the other one

will float. This process, worked out by the E. I. du Pont de Nemours

Company, offers many economies in cleaning coal and concentrating

ores. Do not confuse this process, technically known as "beneficiation,"

with ore flotation which is based on surface tension.

Archimedes' principle is applied in measuring the density of solids

and liquids. One application of this principle is found in the hydrometer.

Hydrometers are floating vessels adjusted for certain ranges of

density. They are buoyed up in proportion to the density of the liquid

measured. The percentage of alcohol in water can be determined, for

example, by the hydrometer, because the density of mixtures of alcohol

and water decreases with increasing concentrations of alcohol. Similarly

the density of sugar solutions is measured with a special type of

hydrometer.

Among the many special uses for hydrometers, the battery-tester is

one that is very familiar to many people. It is based on the fact that

the density of the sulfuric acid solution in a storage battery decreases

as the battery is discharged.

Hydrometers are also used in testing milk and fuel oil and in testing

the purity and concentration of many solutions used in chemistry and

industry.

Submarines are able to float or submerge by changing their weight.

Water is allowed to enter a compartment in the submarine to add to

the weight when it is desired to submerge it. When it is desired to bring

the submarine to the surface again, it is raised by forcing the water

out of the compartment with compressed air.

The density of a solid may be determined by weighing the solid in

the air and then weighing it in water. The difference between the

two weights represents the weight of water displaced by the body, that

is, the weight of an equal volume of water. The ratio of the weight of

the body in air to the weight of an equal volume of water is the density

of the body compared to water.

Convection Currents Result from Unequal Heating of Gases and Liquids.

Inasmuch as gases expand when they are heated, they become less

dense and are, therefore, buoyed up and replaced by denser cold gases,

which rise, in turn, as they are heated. Similar convection currents are

produced in liquids. Very often these convection currents become

visible, because liquids and solids of different densities differ also in

their index of refraction. The index of refraction, as we shall learn

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