70 SCIENCE PRIMERS. [MATERIAL 



force is overcome, and the molecules fly off in all 

 directions as soon as they are un confined. On the 

 other hand, by taking heat away, the repulsive force 

 is diminished, until the molecules become inseparable 

 and the water assumes the solid form. 



It is probable that the expansion of fluid water, at a 

 temperature below 39, depends upon the molecules 

 taking up a peculiar arrangement as they approach 

 one another. If sixteen men are formed into a 

 column, four deep, and each man a foot from the 

 other, the same men may stand closer together and 

 yet form a hollow square, which occupies a larger space. 

 That the molecules of water do take up a particular 

 order in assuming the solid condition, is shown by 

 the crystalline form of ice. Each crystal of hoar- 

 frost owes its shape to the arrangement of its mole- 

 cules, according to a definite geometrical pattern. 



Thus the hypothesis that water is composed of 

 separate molecules, is useful, for it helps us to some 

 extent to explain the properties of water. And, when 

 you study physics and learn the laws of motion, you 

 will find that there is no end to the number of the 

 truths established by observation and experiment, 

 which can be explained by this hypothesis. Hence 

 it may fairly be adopted and employed as a means 

 of picturing to ourselves the order of nature, so long 

 as no facts are discovered which are inconsistent 

 with it. 



49. All Matter is probably made up either 

 of Molecules or of Atoms. 



The same reasons which lead to the adoption of 

 the hypothesis that water is composed of separate 



