=10. The Metric System.=--In order to study the three states of matter with sufficient exactness it is necessary to employ a system of measurement. The system universally employed by scientists is called _The Metric System_. In many respects it is the most convenient for all purposes. Every student should therefore become familiar with it and learn to use it. At the present time, not only do scientists everywhere use it, but many countries have adopted it and use it in common measurements. It was legalized in the United States in 1866. The metric system was originated by the French Academy of Sciences during the latter part of the 18th century. There were so many different systems of weights and measures in use, each country having a system of its own, that commerce was much hindered. It was therefore decided to make a system based upon scientific principles. The length of the earth's quadrant passing from the equator to the pole was determined by surveying and computation. One-ten-millionth of this distance was selected as the unit of length and called a _meter_. Accurate copies of this meter were made and preserved as standards. Later surveys have shown that the original determination of the earth's quadrant was not strictly accurate; so that after all the meter is not exactly one-ten-millionth of the earth's quadrant. =11. The Standard Meter.=--The standard unit of _length_ in the metric system is the _meter_. It is the distance, at the temperature of melting ice, between two transverse parallel lines ruled on a bar of platinum (see Fig. 3), which is kept in the Palace of the Archives in Paris. Accurate copies of this and other metric standards are also kept at the Bureau of Standards at Washington, D. C. Fig. 4 shows the relation between the inch and the centimeter (one-hundredth of a meter). =12. Units and Tables in the Metric System.=--The metric unit of _area_ commonly used in physics is the _square centimeter_. [Illustration: FIG. 3--The standard meter.] The standard unit of _volume_ or capacity is the _liter_. It is a cube one-tenth of a meter on each edge. It is equal to 1.057 quarts. It corresponds, therefore, to the quart in English measure. [Illustration: FIG. 4.--Centimeter and inch scales.] The standard unit of _mass_ is the _kilogram_. It is the mass of 1 liter of pure water at the temperature of its greatest density, 4°C. or 39.2°F. The three principal units of the metric system, the _meter_, the _liter_, and the _kilogram_, are related to one another in a simple manner, since the liter is a cube one-tenth of a meter in each dimension and the kilogram is the mass of a liter of water. (See Fig. 5.) The metric system is a _decimal_ system that is, one unit is related to another unit in the ratio of _ten_ or of some power of ten. This is indicated by the following tables: _Metric Table of Length_