=Definition of Chemical Change.= Chemical changes or processes may be defined as those close and intimate actions amongst the particles of matter by which they are dissociated or decomposed, or by which new compounds are formed, and involving a complete loss of identity of the original substance. =Physical Changes.= Mix a teaspoon of sugar with an equal amount of salt; the sugar is still sugar, and the salt remains salt; and they may each be separated from the mixture as such. Water when frozen is changed from a liquid to a solid; its chemical composition, however, remains unchanged. Water converted into steam by heat is changed from a liquid to a gas, but chemically there is no difference between the one and the other. _Steam_, _water_, and _ice_ are forms of the same substance, the difference being physical, not chemical, and caused by a difference in temperature. Lead melted so that it will run, and the solid lead of a bullet, are the same thing. These illustrate physical changes. =Definition.= When substances are brought together in such a way that their characteristic qualities remain the same, the change is called physical. It is less close and intimate than a chemical change. The transition from one state into another is also frequently only a physical change, as is seen in the transformation of water into steam, water into ice, etc. ELEMENTS One feature of the work of the chemist is to separate compound bodies into their simple constituents. These constituents he also endeavors to dissociate; and if this cannot be done by any means known to him, then the thing must be regarded as a simple substance. Such simple bodies are called _elements_. =Definition.= An element then may be defined as a simple substance, which cannot by any known process be transformed into anything else; that is, no matter how it is treated, it still remains chemically what it was before. Gold, silver, copper, iron, platinum, carbon, phosphorus, calcium, oxygen, hydrogen, nitrogen, and chlorin are examples of elements. Once it was believed that there were but four elements in the world--earth, air, fire, and water. Then it was learned that these were not elements at all, but compounds, and the number of elements increased, until now sixty-eight are admitted to be simple primary substances. Some of these may in the future be proven to be compounds. Sulphur is at present in the doubtful list. =Oxygen.= Oxygen is an element. It is an invisible gas, without taste or smell. It is the most abundant substance in the world, and an exceedingly active agent, entering into nearly all chemical changes and forming compounds with all known elements except one--fluorin. It is a necessity of life and of combustion.[4] It constitutes about two thirds of the weight of our bodies and one fifth of the weight of the air.