Man's physical universe

xanabras

UNIT IV

SECTION 2

A KNOWLEDGE OF THE PROPERTIES OF LIQUIDS

HAS LED TO MANY USEFUL APPLICATIONS

Introduction.

If the temperature could be raised sufficiently, all matter could be

changed to the gaseous state. This is the actual condition of the

matter in the sun. It is also generally recognized that nearly all solids

may be changed to the liquid state by raising the temperature, provided

that they do not decompose or sublime first. The great abundance of

matter in a liquid state (such as water) and the fact that liquids are

more tangible than gases caused some of their properties to be studied

thousands of years ago, and several of the applications studied in this

Section have been known for a long time.

Viscosity.

Liquids possess the property of fluidity, which is a measure of the

ease with which they flow.

Viscosity is a universal property of liquids.

It is the inverse of fluidity, for it is measured by the resistance to flow

The viscosities of such

resulting from the internal friction of a liquid.

liquids as molasses and tar are high at ordinary temperatures, while

the viscosities of water and alcohol are much lower. It is a common

observation that tar is heated in order to make it

is

flow readily, and it

a general rule that viscosities of liquids decrease with increases in

temperature.

Some liquids have such high viscosities that there is a question

whether they are liquids or solids. A true crystalline solid ceases to be

crystalline when it flows. True solids are distinguished from highly

viscous liquids in that true solids have sharp melting-points and

produce characteristic patterns in the X-ray spectrograph. Therefore,

glass tubing which will bend out of shape in a few weeks at ordinary

temperatures and a paraffin candle which droops in the summer

weather are properly regarded as examples of highly viscous liquids.

Lubricating oil is purchased by the S.A.E. number, which refers to

the viscosity measured by standard methods prescribed by the Society

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