Non-luminous Radiation emitted by a Gas Flame. 59 



then it was moved 10 cm. along the scale, and another readiug made ; 

 the distances were thus ascertained at which the whole radiation 

 from the lamp produced a somewhat less and a somewhat greater 

 effect upon the galvanometer than had been produced by that portion 

 of the radiation which had passed through the glass and water of the 

 cell, with the lamp close to the pile. 



The measurements were repeated six times for each position of the 

 lamp ; a curve was then plotted with the distances at which the lamp 

 had been placed when no cell had been interposed between it and the 

 pile as abscissae, and the mean deflections corresponding to each of 

 these positions as ordinates, and then the abscissa of a point whose 

 ordinate was equal to the mean deflection when the cell had been 

 interposed ascertained. This indirect method was adopted, as it 

 would have been impossible, without making a very large number of 

 measurements, to have ascertained the exact position of the lamp for 

 which the direct radiation produced the same effect as that portion of 

 it which had passed through the water. 



Table 1 gives one set of readings made in this way with Cell I, 

 and Table II the - mean of three sets of readings made with the four 

 cells, and with the lamp at different distances from the thermopile, 

 the actual readings being about as concordant as those contained in 

 Table I. 



Table I. 

 Deflection of Galvanometer. 



Mean. 



With cell 



, , 3 5 



3 6 



3'8 



3 7 



3-G 



3-71 







3-7 



3-8 



3-9 



3-6 



3-8 



3-6/ 



3-7 







4-2 



4-1 



4-0 



43 



4-0 



4-2 



120.., 



. . 4 2 



3 13 



3 3 



3-2 



3-6 



3 6 



3 6 



130.... 





3*4 



3 



3'2 



3 



2-8 



3'15 



Table II. 

 Mean Deflection of Galvanometer. 



Mean. 



Cell I 



.... 3-7 



4-0 



4-25 



3-98 



Cell IT 



.. 2'05 



2-27 



2 2 



2-17 



Cell III 



, 1-8 



1-9 



1-8 



183 



Cell IT 





19 



1-9 



1-9 





. , 4-2 



4-13 



4-1 



4-14 



120 



, . , 3 6 



3-7 



3-6 



3-63 



130 



.... 3-15 









160 



, . 2-4 



2 -43 



2-2 



2-34 



170 



. , , 2-0 



2-03 



2 15 



2-06 



180 



.... 1-8 



1-8 



1 -9 



1-83 



190 





1-76 



1-7 



1-72 



In the case of the first set of measurements made with Cell I 

 (those contained in Table I) the equivalent distance for the lamp is 



