Page:International Library of Technology, Volume 93.djvu/80

 As another example, take carbon dioxide, CO2, in which,

1 atom of C × atomic weight, 12 = 12 parts of C 2 atoms of O × atomic weight, 16 = 32 parts of O 44 parts of CO2

Hence, CO2 contains $12⁄44$ = 27.27 per cent, of carbon, and $32⁄44$ = 72.73 per cent, of oxygen. From these examples it is plain that the weight of the molecule, or the molecular weight, of water is 18, and that of carbon dioxide, 44.

9. Definition of Mixture. — Two or more elementary substances may be mixed together and yet not combine to form a new substance. They are then said to form a mixture. The mixture has the properties of the elements composing it. The most familiar example of a mixture is ordinary air. It is composed of oxygen and nitrogen, twenty-three parts by weight of the former to seventy-seven parts by weight of the latter. The two gases do not combine chemically; they are simply mixed.

COMBUSTION

10. Definition. — Combustion is a very rapid chemical combination. The atoms of some of the elements have a very great affinity or attraction for those of other elements, and when they combine they rush together with such rapidity and force that heat and light are produced. Oxygen, for example, has a great attraction for nearly all the other elements. An atom of oxygen is ready to combine with almost any substance with which it comes in contact. For carbon and hydrogen, oxygen has a particular attraction, and whenever these elements come in contact with oxygen, at a sufficiently high temperature, they combine with great rapidity. The combustion in a gas-engine cylinder is of this nature. The temperature of a small portion of the mixture of gas and air is raised by a flame or electric spark, and then the fuel charge begins to combine with oxygen taken from the air. The combination is so rapid and violent that a