§ 463. THE SALTS OF THE SEA. 235 



while certain parts of the earth would have been deluged with 

 rain. Had the seas been of fresh water, the amount of evapora- 

 tion, the quantity of rain, the volume and size of our rivers, would 

 all have been different from what they are ; the quantity of elec- 

 tricity in the air would have been permanently changed from 

 what it is, and its tension in the sky would have been exceeding- 

 ly feeble. In the evaporation of fresh water at normal tempera- 

 tures, but little of that fluid is evolved ; while vapor from salt wa- 

 ter carries off vitreous, and leaves behind resinous electricity in 

 abundance. Hence, with seas of fresh water, our thunder-storms 

 would be feeble contrivances, flashing only with such sparks as 

 the vegetable kingdom might, when the juices of its plants were 

 converted into vapor, lend to the clouds. It might seem strange, 

 this idea that the thunderbolt of the sky, the sheet-lightning of 

 the clouds, and the forked flashes of the storm, all have their 

 genesis chiefly in the salts of the sea, and so it would be held 

 were it not that Faraday has shown that a single grain of water 

 and a little zinc can evolve electricity enough for a thunder-clap ; 

 therefore, were there no salts in the waters of the ocean, the sound 

 of thunder would scarce be heard in the sky* — there would be 

 no Gulf Stream, and no open sea in the Arctic Ocean. 



463. As a general rule, the constituents of sea water are as 

 Uniform character coustaut iu their proportious as are the components 

 of sea water. ^^ x]^^ atmospherc. It is true that we sometimes 



come across arms of the sea, or places in the ocean, where we find 

 the water more salt or less salt than sea water is generally ; but 

 this circumstance is due to local causes of easy explanation. For 

 instance : when we come to an arm of the sea, as the Ked Sea 

 (§ 376), upon which it never rains, and from which the atmos- 

 phere is continually abstracting, by evaporation, fresh water from 

 the salt, we may naturally expect |o find a greater proportion of 

 salt in the sea water that remains than we do near the mouth of 



* The great American lakes afford, it may be supposed, a considerable portion of 

 the vapor which goes to make rain for the hydrographic basin in which they are. 

 Visiting the Lake country in 1858, I was struck with the fact that so few trees bore 

 the marks of lightning. The rule appeared to be, the nearer the lakes, the more rare 

 was it for one of these ornaments of the forest to have been defaced by lightning ; 

 and, on inquiry from the Lake Board of Underwriters, I was informed that among 

 the records of lake disasters there was not a single instance of a vessel having been 

 struck by lightning on the great American lakes ! 



