84 PRACTICAL LESSONS IN SCIENCE. 



ing, water expands with such force as to burst the strongest ves- 

 sels, and break up the most tenacious rocks. The molecular force 

 seems irresistible. The cohesive force is also very strong, and 

 is frequently made use of in mechanical operations, as in setting 

 the tires of wheels, in riveting the plates of boilers, etc. In the 

 building of iron bridges and similar structures provision must 

 be made for the expansion and contraction due to ordinary 

 changes of climate, or the structures would soon go to ruin from 

 the constantly recurring strains due to these forces. 



Glass is a poor conductor of heat; so that if hot water is 

 poured into a thick glass bottle the inner part becomes hot and 

 expands before the outer part gets warm, and the glass is liable 

 to break from the sudden strain. Hence, glass vessels which are 

 to be subjected to extremes of temperature should be thin, so 

 that there may be less chance for unequal expansion. 



Most solid bodies may be changed in form by compressing, 

 stretching, bending, or twisting forces, but when the forces cease 

 to act, in most cases, the body tends to resume its original form. 

 If the body has been compressed, the repellant force corrects the 

 form, while if the body has been stretched, it is the attractive 

 force that does the work of correction. After bending and twist- 

 ing, the same forces attempt to restore the original form. This 

 property of matter by which it assumes its original form after 

 distortion is called elasticity. Many bodies are elastic only 

 within certain limits that is, a body may be strained beyond the 

 limit of recovery the particles may be separated so far that 

 cohesion can not assert its power again. Vegetable and animal 

 substances decay, metals become crystalline and the properties 

 of tenacity, elasticity and strength are gradually lost. 



Liquids are elastic and have other properties in common with 

 solids, but the characteristic property of liquids is mobility. 

 The attractive and repellant forces are so nearly balanced in 

 liquids, that the molecules are allowed to move freely among 

 themselves. Liquids under the action of gravity press down- 

 ward, but their mobility allows them to press sidewise as well as 

 downwards, so that water will pass out through holes in the sides 



