364 Mr. E. H. M. Bosanquet on the Illuminating-power 



be supposed to be less in the mixtures in which hydrogen 

 predominates, and if taken to increase would have to be 

 balanced by a still larger resistance to absorption than that 

 shown. 



The resistance to absorption is measured as ratio of cause to 

 effect*; i. e., it is, 



x x unsaturated capacity/change in illuminating value. 



The constancy of dyjdx generally implies the absence of 

 absorption; but not always; for the diluent may either have 

 its solvent capacity saturated throughout the series, or may 

 continue to absorb, taking always the same proportion of the 



added carbon, so that ~ remains constant. But the existence 

 dec 



of an increase of dyjdx is necessarily connected with the fill- 

 ing up of an unsaturated solvent capacity in the diluent. 



The treatment of the nitrogen-ethylene mixtures is in all 

 respects similar to that of the hydrogen-ethylene mixtures. 

 But the assumption of 201 as the illuminating value of the 

 precipitated carbon becomes doubtful in this case and in that 

 of carbonic acid. In the hydrogen series it appears for 

 several reasons that if the equivalent is not 201, as it is in 

 the combined hydrocarbons of carbon density 2, and in the 

 marsh-gas-ethylene mixtures, it differs but little. But in the 

 nitrogen and carbonic-acid mixtures I cannot see that we 

 have any hold on the illuminating value of precipitated 

 carbon. 



It is curious that the results so far have not seemed to 

 depend on temperature. If we discard temperature altogether, 

 the values of precipitated carbon may yet be the same in the 

 above cases ; but I do not now pursue them. 



In considering mixtures of a diluent having a curved locus 

 with a gas represented by a specified point, the problem of 

 the representation of the mixtures is not generally to be 

 solved by the above materials. For we do not know how 

 the absorption will act under conditions not identical with 

 those of the mixtures we have considered. All we can do is 

 to define as best we can the state of saturation of the gas, 

 noting the point on the y axis towards which a tangent to 

 the curve in that region is directed, and treating that as the 

 best point to represent the diluent for combination with gas 

 of that description. For small corrections this may give an 

 approximate idea. 



I have determined the points I use for coal-gas by laying 



* For this definition of "resistance," see my paper "On the Use of 

 the term 'Resistance' in the Description of Physical Phenomena," Phil. 

 Mag. May, 1888, p. 419. 



