789. _What is the specific gravity of a body?_ It is its weight estimated _relatively to the weights of other bodies_. 790. _What determines the force with which bodies fall to the earth?_ Generally speaking, their _specific gravity_, which is proportionate to the density, or _compactness of the atoms_ of which they are composed. 791. _Why does a feather fall to the earth more gradually than a shilling?_ Because the _specific gravity_ of the feather and of the shilling is _relative to that of the air_, the medium through which the feather and the shilling pass. If there were _no air_, a shilling and a feather dropped at the same time from a height of forty miles, _would reach the earth at the same moment_. CHAPTER XXXVII. [Verse: "Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding."] 792. _What is repulsion?_ Repulsion is that property in matter by which it _repels_ or _recedes from_, those bodies for which it has _no attraction or affinity_. 793. _Why does dew form into round drops upon the leaves of plants?_ Because it _repels the air_, and the _substances of the leaves_ upon which it rests. Because, also, its own particles _cohere_. 794. _Why do drops of water roll over dusty surfaces?_ Because they _repel_ the particles of dust; and also because their own particles have _a stronger attraction for each other_ than for the particles of dust. 795. _Why does a needle float when carefully laid upon the surface of water?_ Because the needle and the water _mutually repel each other_. 796. _Why does water, when dropped upon hot iron, move about in agitated globules?_ Because the _caloric_ repels the particles of the water. 797. _Why does oil float upon the surface of water?_ Because, besides being specially lighter than water, the particles of the oil and the water _mutually repel each other_. 798. _What is carbonic acid?_ Carbonic acid is a mixture of _carbon_ and _oxygen_, in the proportion of 3 lbs. of carbon to 8 lbs. of oxygen. [Verse: "Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it?"] 799. _Where does carbonic acid chiefly exist?_ It exists in various natural bodies in which carbon and oxygen are combined; it is evolved by the decomposition of numerous bodies called carbonates, in which carbon is united with a particular base, such as the carbonate of lime, the carbonate of iron, the carbonate of copper, &c. It is also evolved by the processes of _fermentation_, by the _breathing of animals_, the _combustion of fuel_, and the _functions of plants_. Carbonic acid also _exists in various waters_. Carbonic acid is _found most largely in solid combinations with other bodies_: it forms 44-100ths of all limestones and marbles, and it exists in smaller quantity, combined with other earths, and with metallic oxides. 800. _What are the states in which pure carbonic acid exists?_