1855.] 



NOTE ON THE OBJECT OF THE SALT CONDITION OF THE SEA. 



187 



Cyclidiae, and OphrydinEe. The waters swarm with countless 

 hosts of small luminiferous animalcules, Mammaria (of the 

 order Acalephse), Crustacea, Pteridinia, and circling Nereides, 

 which, when attracted to the surface by peculiar meteorological 

 conditions, convert every wave into a foaming band of flashing 

 light."* These creatures preying upon one another, and being 

 preyed upon by others in their turn, the circulation of organic 

 matter is kept up, and carried through its appointed rounds. 

 If we do not adopt this view, we must at least look upon the 

 animal infusoria, the foraminifera, and many other forms of 

 higher types of organisation, as scavenger agents appointed to 

 prevent an undue accumulation of decaying matter ; and in 

 either case, so far as regards the object under discussion, the 

 oceanic waters might have been as well fresh, as salt. 



According to the second suggestion, the sea holds saline 

 matter in solution, in order to render it of greater density. The 

 superior density of sea-water as conipared to fresh, undoubtedly 

 plays an important part in many of the physical phenomena of 

 which the ocean is the stage. A greater counterpoise is thus 

 necessarily ofiered to lunar and other cosmical attractions ; and 

 the effects produced by winds and atmospheric disturbances 

 must be modified also by this principle to no slight extent; but 

 causes such as these, when considered in their fullest relations, 

 can scarcely be considered adequate to meet the entire solution 

 of so vast and grand a problem as that which is manifestly 

 involved . in the salt condition of the sea. Neither can the 

 third supposition, as given above, be considered of greater value 

 in this respect, because the difference between the freezing 

 points of fi-esh and sea-water is under 4° Fah. ; and hence, 

 with the present distribution of land and water, and still less 

 probably with that of former geological epochs, no very impor- 

 tant effects would have resulted from this cause, if the ocean 

 had been fresh instead of salt. So far as regards the habitable 

 portions of the world for instance, the present difference would 

 be next to nothing. I do not mean to imply, nevertheless, 

 that this principle may not be without some secondary bearings 

 on the phenomenon in question ; but I do not consider it sul- 

 ficient for the complete elucidation of the same. 



The suggestion which I have now to lay before the Institute, 

 as an attempt to explain the object in view — although con- 

 fessedly not free from certain difficulties — will be found, I 

 think, of a far more satisfactory character than those hitherto 

 advanced. As already mentioned, I am not aware of any pre- 

 vious application of the principle which it embodies, to the so- 

 lution of the present question. Without further preface, then, I 

 may state that I regard the salt condition of the sea as maiulij 

 intended to regulate evaporation, and to prevent within certain 

 limits, an undue excess of that phenomenon under the influence 

 of any disturbing causes that might from time to time arise. 

 It has been long known that different liquids boil at the same 

 atmospheric pressure, under very different degrees of tempera- 

 ture ; and that, of two saline solutions, the more strongly 

 saturated requires the higher temperature to be raised to the 

 boiling point. In like manner, evaporation at natural tempe- 

 ratures, other conditions being equal, proceeds far more slowly 

 from saturated than from weak solutions; and, necessarily, 

 more slowly also from these latter, than from ordinary water. 

 In sea-water we have, as a mean, about three and a half per 

 cent, of solid mattei-s : 2-6 of this, on an average, consisting 

 of chloride of sodium. 



* Cosmos, vol. 1. 



In order to observe the effects produced in retarding evapo- 

 ration by so small a quantity of solid matter in solution, but 

 without attempting to imitate the complex composition of sea- 

 water, I placed a weighed quantity of ordinary rain water, and 

 the same holding in solution 2-6 per cent, of Na CI, in por- 

 celain vessels of equal diameter; and exposed the two, side 

 by side, to spontaneous evaporation : re-weighing them every 

 twenty-four hours for six days. The experiment was then 

 repeated, but with an exchange of vessels, so as to eliminate 

 any errors that might arise from a slight difference in the 

 diameters of the capsules employed. The results of each set of 

 experiments were strikingly in accordance. The mean results 

 of the two weighings, reduced to their per centage quantities, 

 are given in the annexed table. Column A shews the evapo- 

 ration loss resulting from the water. Column B, the same from 

 the water of the salt solution : the amount of salt being de- 

 ducted throughout as a constant quantity, and its weight con- 

 firmed by evaporation and re-weighing at the close of the 

 experiment. Column C exhibits the excess of evaporation of 

 A over B. 





A 



B 



C 



HOUES. 



Loss from the 



Loss of Water from 



Excess of loss of 





Rain Water. 



the Salt Solution. 



A over B. 



24 



8.83 



8.29 



.54 



48 



19.12 



18.08 



1.04 



72 



26.63 



25.17 



1.46 



96 



30.82 



29.05 



1.77 



120 



39.00 



36.76 



2.24 



144 



44.99 



42.43 



2.56 



An excess of 0.54 per cent, in twenty-four hours, in the 

 evaporation of fresh water over water containing 2.6 per cent, 

 of Na CI, may seem ;:t first sight of little moment; but when 

 we consider that this arose from a surface under two inches 

 square, we may easily conceive how enormous would be the 

 difference between surfaces of fresh and salt water so vast as 

 that of the present ocean, even ; and so exposed over wide areas 

 to evaporation-tending influences. Besides which, it must be 

 remembered that the salt solution of the above table, contained 

 at the commencement of the experiment, 1 per cent, less of solid 

 matter than that present in the waters of the sea. It will be 

 remarked, that as the salt solution becomes more and more 

 concentrated, the excess of evaporation of A over B becomes 

 higher and higher. 



Here then we have a self-adjusting phenomenon ; one of 

 those admirable contrivances in the balance of forces, which an 

 attentive study of nature reveals to us in every direction. If, 

 other condit'ons being the same, any temporary cause render 

 the amount of saline matter in the sea above its normal value, 

 evaporation goes on the more and more slowly ; and, on the 

 other hand, if this value be depreciated by the addition of fresh 

 water in undue excess, the evaporating power is the more and 

 more increased; thus aiding time, in either instance, to restore 

 the balance. 



In conclusion I would observe, that the consideration of this 

 principle may shed some further light on the geographical dis- 

 tribution of fresh and salt-water lakes on the present surface of 

 the globe. 



