190 SOILS. 



most vital not only to living organisms, but to the entire econ^ 

 omy of Nature. 



Density. As regards the density or specific gravity of water 

 (which is by common consent assumed as the unit of com- 

 parison), it will be seen from the " Density " table that whereas 

 all other bodies contract and become more dense as they grow 

 colder, water has its point of (fluid) " maximum density " at 

 4 C. (49. 2 Fahr), and expands as it grows colder, until at 

 o C. (32 Fahr.) it solidifies into ice. In so doing it de- 

 parts still farther from the rule obtaining \vith all other bodies 

 (excepting certain mixtures, such as type metal) and again ex- 

 pands so as to decrease the density from .99988 to .92800 ; thus 

 causing ice to float on water at the freezing point. Hence 

 water, unlike all other fluids, solidifies first on the surface ; and 

 but for this, the thawing of the winter's ice, which would be 

 formed at the bottom of rivers and lakes, would be deferred 

 until late in summer. The expansion of water in freezing is 

 forcibly illustrated in the bursting of water pipes and pitchers 

 in winter; in the soil, the ice forming in the interstices serves 

 to loosen the compacted land and give it better tilth for the en- 

 suing season. 



Specific Heat. Considering next, the column showing the 

 " specific heat " of water as compared with other substances, we 

 see that it exceeds all other known bodies in the amount of heat 

 required to change its temperature; hence again, its heat capa- 

 city is taken as the unit to which all others are compared. The 

 figures given in the table show that even ice and steam require 

 for equal weights only about half as much heat (or burning of 

 fuel) to change their temperature (c. g., i degree) as would 

 liquid water. But earthy matters, such as clay or soil and 

 glass, require only one-fifth as much heat for a similar change; 

 charcoal only about one-fourth as much. But vegetable mat- 

 ter as represented by wood on the one hand, and gold and lead 

 on the other, require only about one-thirtieth as much heat as 

 an equal weight of water; zinc about one-tenth as much, steel 

 somewhat more. 



It is thus plain that masses of water act powerfully, more 

 than any other substance, as moderators of changes of temper- 

 ature by their mere presence. The body of an animal or 

 plant is protected against violent changes by the presence of 



