and the gas-the latter, in the strict sense, signifying matter which cannot be converted into a liquid or solid by the coldest climate, thus differing from a vapour, steam for example, which, by exposure to cold, returns to the liquid state, the process being termed condensation. 7. All bodies are composed of minute portions, termed atoms, which can only be separated by a chemical process. A mechanical one-grinding, termed trituration, for example-can reduce a body to the finest power, but not to atoms-for its particles can still further be divided by solution, which is a chemical process. At this point we arrive at the limit of divisibility, which means the capability of being divided. This is a universal property of matter. QUESTIONS. 22. In how many states is matter presented to our notice? 23. How does a gas differ from a vapour? 24. What is the difference between an atom and a particle? 25. Can we reduce a body to atoms by grinding? 26. What is the definition of divisibility? 8. Another is impenetrability, which implies that two bodies cannot occupy the same space at the same time. A familiar illustration of this law is inverting a wine glass in water; the air that occupies the inverted vessel will not allow the surrounding water to take possession of the space: upon this principle the diving-bell is constructed. 9. Impenetrability yields to a certain extent to another, compressibility, which produces some of the most familiar effects. In the case of the inverted wine-glass although the air does not leave the vessel, it is forced into a smaller space by the pressure of the water above it, particularly if it be immersed to any considerable distance from the surface. It has been ascertained by means of the force-pump that air is so compressible that a hundred parts, say pints, can be forced into the space of one pint. This is technically expressed thus: air can be compressed into the hundredth part of its original volume. When alluding to air in this state the term condensed is substituted for compressed. It is to be particularly remembered that the term compressed, or high pressure, applied to steam, means that it occupies a smaller space without changing its condition-while condensed signifies that it has returned to the state of water.-(Vide 6.) 10. When bodies are compressed, another property, elasticity generally comes into operation, tending to restore them to their original condition—the air-gun offers an excellent illustration-the mechanism of which is easily explained :-condensed air confined in a copper vessel, being allowed to rush, by its elastic force, into the barrel of the gun, it projects the bullet to a greater or less distance, according to the extent of its condensation; compressed steam, by the same principle, will burst the strongest vessel, if not allowed to escape by a safety-valve. QUESTIONS. 27. What property of matter prevents water from completely entering into an inverted wine-glass? 28. Why does it do so partially ? 29. To what extent can air be condensed? 30. What is the difference between condensed air and condensed steam? 31. What property of matter allows the condensed air in the air-gun to propel the bullet? ATTRACTION. THE next property we have to consider requires particular attention; it is termed attraction, from the Latin attraho, to draw towards: observation and experiment have proved that it is an invariable law of nature, for bodies to attract, or tend to attract, other atoms of a similar or dissimilar nature. Without this unseen but mighty power, the earth would crumble into dust, and all its bodies be whirled into boundless space. According to the mode in which attraction acts, it receives different terms; for example, attraction of cohesion and gravitation; chemical attraction; magnetic, electrical, and capillary attraction in the present work the three first will be noticed. 12. Attraction of cohesion (derivation cohæreo, to be joined together,) implies the property which the atoms of matter possess, of attracting atoms of the same nature. Although it is an universal law, the Creator has wisely ordained that it should exist in В |