28 CHEMICAL PHYSICS. 



Vapors from liquids and solids at sufficient temperatures above their 

 points of condensation behave just like gases. 



Divisibility. All matter admits of being subdivided into smaller 

 particles, and this property is called divisibility. The processes by 

 which we accomplish the comminution of a solid substance may 

 be of a mechanical nature, such as cutting, crushing, grinding ; but 

 beside these modes of subdivision we have other agents or causes 

 by which matter may be divided into smaller particles, and one of 

 these agents is heat. 



Action of heat on matter. Let us take a piece of ice and 

 convert it, by means of mortar and pestle, into a very fine powder. 

 When the smallest particle of this finely powdered ice is placed 

 under the microscope and heat applied, we shall observe that it 

 becomes liquid, thus proving that it was capable of further sub- 

 division, that it consisted of smaller particles, which have now by 

 the action of heat become movable. By further applying heat to the 

 liquid particle of water we may convert it into a gas or vapor, which 

 will escape into the air, or which we may collect in an empty flask. 

 The flask will be filled completely by this water-gas (or steam) 

 obtained by vaporizing that minute particle of ice-dust. This fact 

 demonstrates that mechanical comminution does not carry us beyond 

 a certain degree of subdivision of matter. That is to say, the smallest 

 fragment of the finest powder still consists of a very large number of 

 much smaller particles. To the smallest particles which compose 

 matter the name molecules has been given. 



Molecular theory. The expression molecule is derived from the 

 Latin word molecula a little mass, and means the smallest particle 

 of matter that can exist by itself, or into which matter is capable of 

 being subdivided by physical actions. To explain more fully what is 

 meant by the expression molecule, we will return to the conversion of 

 water into steam. 



When water boils at the ordinary atmospheric pressure it expands 

 about 1800 times, or one cubic inch of water yields about 1800 cubic 

 inches, equal to about one cubic foot of steam. In explaining this 

 fact we have either to assume that the water, as well as the steam, is 

 continuous matter (Fig. 13), or that the water consisted of small par- 

 ticles of a given size, which now exist in the steam again as such, 



