194 Profs. Richardson and Cooke on the Heat developed 



■with oxygen are marked thus ®, the points for the grid 

 saturated with hydrogen thus ; while the points with the 

 platinum wire grid are shown thus £. As the diagram is 

 constructed, the points appear to fall into two groups, each 

 lying very near a straight line. This is because the hydrogen 

 group has been separated from the oxygen group by shifting 

 its scale of voltage four units to the right so that for the 

 points which fall in the right hand group the voltages are 

 less than the reading on the scale of abscissa? by 4 volts.' 

 The points for the platinum wire grid have been plotted 

 twice, once on each of the two voltage scales mentioned. 

 Thus they fall near to the line drawn through the oxygen 

 points as well as to the line drawn through the hydrogen 

 points. It will be seen that the oxygen points fall on the 

 straight line with exceedingly close accuracy, and that the 

 platinum wire points also fall on the same line with about 

 the same accuracy. As has already been observed, the 

 hydrogen points are not so consistent among themselves and 

 the best line through them falls a little off the best line 

 through the platinum wire points. When corrected for the 

 joulage and for the temperature energy of the electrons, 

 the best line through the oxygen points on this diagram 

 gives for the mean value 5*45 volts, while the best line 

 through the hydrogen points gives 5*05 volts. The value 

 obtained in this way for oxygen is somewhat less and for 

 hydrogen somewhat greater than the arithmetical mean of 

 the observations previously obtained. But they agree in 

 indicating that the effect when the filament has been satu- 

 rated with hydrogen is lower than the value in oxygen. 



It is remarkable that all the observations on the platinum 

 grid saturated with oxygen and the platinum wire grid, which 

 have been used in deducing the values given in the previous 

 table, should fall so nearly on the straight line as they are 

 seen to do from fig. 5 (PI. III.). It will be recollected that 

 the individual values for the effect recorded in those tables 

 showed considerable variation. This emphasizes the im- 

 portance of taking every observation with great care and 

 accuracy since it is clear that a small error in the position of 

 the line makes a very large percentage error in the final 

 value of the effect. 



The position of the points in figure 4 is corrected for the 

 difference in the potential drop along the filament in the 

 different experiments, but no correction has been made on 

 account of the difference in the heating effect of the Wheat- 

 stone's bridge current in the different cases. It is not 

 believed that the omission of! this correction has made any 



