Because the soap unites with the mineral matters in the water, and being _neutralised_ thereby, cannot _dissolve the dirt_ which we desire to cleanse away. 764. _Why is the sea salt?_ Because salt is a mineral which prevails largely in the earth, and which, _being very soluble in water_, is taken up by the ocean. Lakes and rivers, also, even those that are considered fresh, hold in solution _some degree of saline matters_, which they contribute to the ocean. As, in the evaporations from the sea, the salt remains in it, while the vapours fall as rain, and again wash the earth and carry some of its mineral properties to the ocean, the _greater saltness of the sea_, as compared with rivers, is accounted for. By some persons the opinion is entertained that the sea has been _gradually getting salter_ ever since the creation of the world. This, they say, arises from the evaporation of water free from salt, and the returns of the water to the sea, taking with it salt from the land. 765. _What is the estimated amount of salt in the sea?_ The amount of common salt in the various oceans is estimated at 3,051,342 _cubic geographical miles_, or about five times more than the mass of the mountains of the Alps. 766. _What is the depth of the sea?_ The extreme depth has not, probably, been ascertained. But Sir James Ross took soundings about 900 miles west of St. Helena, whence he found the sea to be nearly _six miles in depth_. Now, if we take the height of the highest mountain to be five miles, the distance from that extreme rise of the earth, to the known depth of the sea, will be no less than _eleven miles_. 767. _Why are the waters of some springs impregnated with mineral matters?_ Because the water passes through beds of soda, lime, magnesia, carbonic acid, oxides of iron, sulphate of iron, &c., &c., and _takes up in some slight degree the particles of those minerals_, according to the proportions in which they abound. [Verse: "Who hath measured the waters in the hollow of his hand, and meted out heaven with the span, and comprehended the dust of the earth in a measure and weighed the mountains in scales, and the hills in a balance?"--ISAIAH XL.] 768. _Why does iron rust rapidly when wetted?_ Because the water contains a large proportion of oxygen, some of which combines with the iron and forms _an oxide of iron_, which is _rust_. 769. _Why does stagnant water become putrid?_ Because the _large amount of oxygen_ which it contains accelerates the decomposition of dead _animal and vegetable substances_ that accumulate in it. 770. _Is there danger in drinking water on account of the living animalcules which it contains?_ No danger arises from the _living creatures_ in water; but _putrefactive_ matters may produce serious diseases. 771. _What is the best method of guarding against impurities?_