MEASUREMENT OF SALINITY OF SEA WATER 13 



40.22 curves, curves for sea Avater of salinities 31, 32, 33, 34, 35, 36, 

 and 37 parts per thousand were obtained; by interpolation between 

 the distilled water and the 5.34 curves, curves were obtained for 

 boiler water of salinities 0, 1, 2, 3, 4, and 5 parts per thousand, corre- 

 sponding to 0, 32, 65, 97, 129, and 162 grains chlorine per gallon. 



These sea-water and boiler-water curves are given in Figures 5 

 and 6, respectively. Correspondence with the manufacturers indi- 

 cates that calibration charts applying to one dipping refractometer 

 will ajDply to any refractometer of the same type within about two 

 units in the third figure of salinity, provided the instruments are 

 adjusted to agree on some one sample at a given temperature. 



So far as it has been possible to test the curves before this publica- 

 tion was prepared, they have given salinities in fair agreement with 

 results obtained by chemical titration. The curves should be re- 

 garded as preliminary, however, because they are not based upon a 

 large variety of samples. Extended experiments with the refrac- 

 tometer to a large number of samples, which will be standardized by 

 the pycnometer method, will doubtless give more precise curves. 



The following procedure is recommended for the use of the dip- 

 ping refractometer. In fairly smooth weather so that it is feasible, 

 the instrument should be used in connection with the heating trough 

 and small glass cups supplied with the instrument, because such pro- 

 cedure insures more accurate temperature determination. The trough 

 should be filled with fresh water, at about room temperature, nearly 

 to the top of the overflow pipe. Two of the cups should be filled 

 about half full of the sample, after being well rinsed with sample, 

 and placed side by side in the center rack. The prism of the refrac- 

 tometer should be dipped into one cup and then placed in position in 

 the other cup ready for observing. The thermometer should be sus- 

 pended in the cup not occupied by the prism. 



After allowing the instrument to stand for two or three minutes, 

 for the temperature of the two portions of sample to come to that of 

 the water bath, a series of 10 observations should be taken of the 

 position on the scale of the chord separating the dark and illuminated 

 portions of the field, the thermometer being read on the first, fifth, 

 and tenth scale readings. The proper graduation on the scale should 

 be brought into coincidence with the chord of separation alternately 

 from above and from below. 



From the mean of the 10 scale readings and the mean of the three 

 temperature readings the salinity of the given sample can be read 

 off the calibration chart. The observer looks out the point on the 

 chart corresponding to the mean scale reading and the mean tem- 

 perature. If, for example, this point falls seven-tenths of the dis- 

 tance, measured parallel to the axis of scale readings, from the 



