556 SCIENCE PROGRESS 



the manometer. The meniscus error thus determined is only 

 about three-fourths of that calculated for spherical surfaces ; 

 this may be due to the actual shape of the meniscus or perhaps 

 to a tendency to read the columns too short ; in either case the 

 same factors would probably appear in the reading of the 

 manometers and would be eliminated by taking the corrections 

 as found experimentally rather than by calculation. The 

 correction for the meniscus amounts to o*i4i per cent, in 

 decinormal solutions increasing to 1*07 per cent, in normal solu- 

 tions ; but it is believed that the difference of 25 per cent, between 

 the experimental and the calculated corrections is much greater 

 than the actual error in this correction ; in any case the 

 meniscus error is insignificant when dealing with temperature 

 coefficients. 



The capillary depression of the mercury was determined by 

 direct comparison of the readings in the tube with those of a 

 wide tube into which mercury was driven up from the same 

 reservoir. The correction amounted to as much as 18 mm. or 

 o - o23 atmosphere and was one of the most fertile sources of 

 error, since no relationship could be traced between the varia- 

 tions of capillarity and variation of bore. The same apparatus 

 was used to determine the volume of purified nitrogen finally 

 introduced into the manometer tube after sealing on to the bulbs, 

 etc., shown in fig. 3. In each case it was found that increased 

 errors appeared when using a calibrated tube or manometer 

 as a standard for direct comparison : in this case the readings 

 were affected by errors due to the irregular capillarity in both 

 instruments and it was found desirable (in spite of the increased 

 labour involved) to regard each manometer as an independent 

 standard. The labour involved in this essential and difficult 

 work is illustrated by the statement that " the whole time of 

 one of the authors of this paper is given up to the study of the 

 manometers which have been or are to be used in our measure- 

 ments of osmotic pressure." 



3. The Regulation of Temperature. — Questions of exact regula- 

 tion of temperature are of altogether exceptional importance in 

 the measurement of osmotic pressure. In nearly every kind of 

 physical work it is sufficient that uniformity of temperature 

 shall prevail throughout the apparatus at the moment when 

 the readings are taken. But in dealing with osmotic 

 pressure any temporary fluctuation of temperature produces 



