The oxygen-content of sea-water may be estimated from the P„, since the differences between cubic centimeters of CO, and in 2 samples of water of the same excess base are reciprocal.
The change in P,, with change in tension is affected only by the temperature, as shown in conversion table in figure 6.
Changes in salinity theoretically affect the tension and hence content of sea-water. Fox divides the into two parts, one of which obeys Henry's law and never exceeds 1 per cent of the total. It is this small fraction that is affected by salinity, but the changes in salinity ordinarily met with in the larger oceans change this fraction only about 1.5 per cent, and the total change is exceedingly small. We have not been able to determine any effect of salinity on P. or content.
Henderson and Cohn state that if the tension is maintained constant (at about 0.0004 atmosphere) the P. changes from 8.03 at 30.73 salinity to 8.1 at 34.6 S. We were unable to confirm this result with the hydrogen electrode. Henderson and Cohn used the indicator method of Sorensen and Palitzsch, and the difference between succes sive standard tubes as recorded in several of the papers of these authors is P.= 0.1. The tubes were exposed to the air, but were occasionally renewed. Their results were sometimes expressed in figures of four decimal places, but this probably was not intended to have more than mathematical significance. In fact, we found that, with the most accurately measured sealed tubes placed in a colorimeter that brings the centers of the tubes together on a sharp line, we could not dis tinguish with certainty any difference smaller than about P. 0.02, even in the most sensitive region of the range of the most brilliant indicators. The fact that Henderson and Cohn observed P. 8.06 at S 31.58, 8.07 at 32.05, and 8.06 at 32.45, indicates that they made small errors in the estimation of P. or tension or temperature. Sorensen and Palitzsch give the salt error of their indicators at 20 and at 35 salinity, and Henderson and Cohn may possibly have attempted to estimate the change in salt error from S 30.7 to 34.6, but they do not mention the fact.
We have recalibrated our salt errors about 50 times, but the different determinations do not coincide within an error of less than 0.025 P. On the average it was found to be necessary to subtract 0.05 from the observed indicator P. (sulfophthalein series) when the salinity was increased from 30 to 37.7 in order to obtain the same result as with the hydrogen electrode. If this correction is applicable to Henderson and Cohn's indicator for this range (probably phenolphthalein) the increase in P. with rise in salinity from 31.58 to 34.6 is reduced to 0.2 and is about the limit of the smallest possible difference that can be determined with the eye, whereas the range of salinity is as great as is ordinarily met with in the larger oceans. We feel safe, therefore, in assuming that the influence of salinity on the relation of tension to P. at any specified temperature in negligible. In fact, Henderson and Cohn do not state the variation in temperature. In our experi ments the temperature was observed within 0.05° and controlled within 0.2° during the majority of the experiments and to within 0.1° during critical periods.
In figure 6, the curves for 30°, 20°, and 10° are taken from the means of many determinations, whereas the curve for 0° was extra polated. The results of Henderson and Cohn's experiments at 20° are marked by crosses. Their results at higher tensions more nearly fall on a smooth curve, but it is impossible to compare them with hydrogen-electrode determinations, because the hydrogen electrode is not very reliable in low partial pressures of hydrogen, even though a correction be applied for change in hydrogen pressure or concentration.
By means of the conversion table (fig. 6) it is possible to determine the CO2 tension of sea-water from the temperature and P. as read by means of the standard tubes and colorimeter described on pages 44-46. The tension is of first importance in the respiration of marine organisms.