of the Fishery Board for Scotland. 



449 



Further, if this bo true, it follows that the salinity, that is the percentage of 

 total salts in any given sample of sea water, may be calculated with per- 

 fect accuracy from the simple determination cither of its specific gravity 

 or of the percentage of chlorine which it contains, and also, of course, that 

 the percentage of chlorine may be calculated from the specific gravity and 

 vice versa. Professor Dittmar in his report on the composition of ocean 

 water expresses this latter assumption by the formula 



X 



where : J$ t is the specific gravity of the sea water at temperature t (water 

 at 4° C. = 1000) and 4 W t the specific gravity of pure water at t° (water at 

 4° C. = 1000). 



X = Chlorine in 1000 parts, or more correctly the total halogen (chlorine, 



bromine, and iodine) calculated as chlorine. 

 D = Constant. 



A simpler expression is — 



fit - 1000 _ /Pure water at f\ 

 \ =100 ° J 



Supposing this uniformity of composition to hold in all strictness 

 for true ocean waters, it is obvious that it must be disturbed near the 

 mouths of rivers, which are known to contribute a continual supply of 

 soluble matter composed of salts mixed in proportions altogether 

 different from those in sea water. 



In examining the waters of the firths and estuaries along our coast, it 

 appeared to me to be a problem demanding special attention, in view of 

 its possible bearings upon marine biology and questions generally con- 

 nected with the life history of our food-fishes, to determine the extent to 

 which the relative proportions of the salts in our firths and estuaries 

 differ from that characteristic of the water of the open sea. 



At first sight it might appear that the most direct way of arriving at a 

 solution of this problem would have been to make full analyses of large 

 samples of water collected from a few selected localities. To this course, 

 however, there are serious objections. In the first place, from a simple 

 consideration of the labour involved by each such complete analysis, it was 

 certain that a very great expenditure of time and money would neces- 

 sarily be incurred before anything like a satisfactory knowledge could 

 be gained of the quantitative composition of the saline matter in the 

 waters of any one firth or estuary. 



A still greater objection appears when it is remembered that the magni- 

 tude of the differences in the quantitative composition of the soluble 

 matter of such waters would be in all probability not un frequently very 

 small, so that the unavoidable errors attaching to all analytical determina- 

 tions might, in some cases at least, more than counterbalance them. This 

 would be especially likely to occur in the determinations of the variations 

 in the relative quantities of such constituents as lime, magnesia, carbonic 

 acid, and the dissolved gases, which are precisely those of more immediate 

 importance with respect to marine biology and in which it is difficult to 

 attain to the highest degree of precision. The accuracy of the analytical 

 determinations of the different constituents of the soluble matter in sea 

 water is by no means equally great. Thus the most skilful analyst will 

 not be able to estimate the magnesia present in sea water with a degree of 

 accuracy at all equal to that to which he can attain in the case of the total 

 halogen. 



I therefore adopted from the first the method which has been fully de- 

 * Physics and Chemistry of the Yoyage of H.M.S. Challenger, part. i. 



