538 THE POPULAR SCIENCE MONTHLY. 



pound, as sulphuric acid and the four basic principles, soda, magnesia, 

 lime, and potash. Chlorine is by far the most abundant principle, and 

 should be credited with more than half the weight of the saline mat- 

 ter. Open any book on chemistry or the physics of the globe, and you 

 will find that sea-water contains, by the litre, so much chloride of so- 

 dium, so much sulphate of magnesia, so much chloride of magnesium, 

 etc. These affirmations are wholly hypothetical, for our acquaintance 

 with chemistry is not sufficient to permit such conclusions. Analysis 

 shows that there are in a litre of sea-water so much chlorine, so much 

 sulphuric acid, and so much magnesia, but does not reveal to us how 

 the radicals are united, for the combinations we get in analysis are not 

 probably precisely the ones that existed in the water. We might say 

 that the numerous simple bodies entering into the composition of sea- 

 water are all the time contracting nev/ and incessantly variable alli- 

 ances, according to the temperature or the concentration of the liquid. 

 It is by intelligently utilizing these laws that they succeed at the salt- 

 works in forcing the mother-waters to deposit at one time cooking-salt, 

 at another time some other combination useful in industry, or which it 

 is desirable to get rid of. 



In evaporating to dryness a known quantity of sea-water, under 

 certain precautions, we obtain a residue which, well dried and weighed, 

 furnishes the weight of the total quantity of salts originally dissolved. 

 It is then easy, by a simple calculation, to estimate the proportion of 

 solid substances contained in a litre. Salt-water is denser than fresh 

 water of the same volume and temperature, and this excess of density is 

 evidently proportional to its richness in saline matters. This can be 

 obtained by multiplying the excess of density by 1*32. We may thus 

 replace the chemical operation by a determination of density, an easier 

 experiment, and one that can be made on board ships. 



The different oceanic regions are not equally rich in salts. What 

 we have said respecting variations of specific weight shows this very 

 clearly. But, if we always draw the water from a sufficient depth, 

 the variations become much less, as Forchhammer has proved. The fig- 

 ures in his tables oscillate between thirty-four and thirty-five grammes 

 per litre. The relative proportion of the different elements is still more 

 invariable, and we can establish a few slight differences only by taking 

 the means of a large number of estimations. This fixity of relation 

 might have been foreseen, because evaporation concentrates, without 

 taking away an atom of salt, while fresh waters dilute without furnish- 

 ing any. It folloAvs, then, that the composition of a specimen of sea- 

 w^ater can be estimated with a fair degree of accuracy by ascertaining 

 the proportion of one of its constituents, the chlorine, for instance, and 

 that element is much used as a standard. The amount of chlorine in 

 a litre of liquid collected along the shore diminishes obviously when 

 the ship is appi'oaching ice, or if it is cruising near the mouth of a 

 great river. 



