12 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In particular the quartz arc spectrum displays a very striking gap 

 between wave length 334.14 ixjj. and the double line at wave length 



313.1 fxjji. Save for the very faint haze of continuous spectrum that 

 characterizes the radiation from the quartz tube this part of the 

 spectrum is blank. Indeed the line 334.14 jxtx itself is far from strong 

 relatively to those in the further part of the ultra violet and there is 

 very little effect of radiation between wave length 313.1 /z^t and 



365.2 /x/i. This gap is of some significance in interpreting the results 

 of bactericidal experiments, since any failure of bactericidal action in 

 the region between wave length 350 mx and wave length 313 /xju 

 observed in working with the quartz lamp may be due to the absence 

 of any strong radiation in this region as well as to lack of specific 

 bactericidal power in rays of this particular wave length if they existed. 



In the radiometric investigations on the old quartz lamp it was run 

 at 3.7 amperes and about 80 volts, an average of about 260 watts, 

 without an external globe. Under these circumstances the corrected 

 deflection due to the total ultra violet was 16.7 cm. The deflections 

 were not quite so steady as in the case of the ordinary Cooper Hewitt 

 tube, but still the average departure of a single reading was within 1%. 

 After the deflection due to the total ultra violet was determined 

 another set of readings was taken with the bare lamp and quartz 

 cell and then with the Euphos glass replaced by the crown glass; 

 of which the absorption spectrum is shown at /, Plate 1. 



This glass in effect cuts off substantially the whole of the extreme 

 ultra violet spectrum, letting pass in practically undiminished strength 

 only the lines of greater wave length than 300 ixjx. This separation 

 is of some importance with respect to the bactericidal power of the 

 lamp in water purification and similar work. The result was to show 

 that the transmission of the crown glass was 54.7 % of the transmission 

 found for the Euphos glass. In other words, nearly one half of the 

 total ultra violet energy in this lamp was of wave length below 300 nix. 

 Of the remaining half the spectrum shows, as just indicated, that by 

 all odds the larger part lies between 365 MM and the visible spectrum. 



The new cjuartz lamp without its globe was then tested, the input 

 in this case being 350 watts. The ultra violet output was greater 

 than in the old tube, the total deflection reduced to the standard 

 distance rising to 32.1 cm. In this case 65.1 % of the energy trans- 

 mitted by the quartz water cell was cut off by the Euphos glass. 

 Following up the radiometric measurement further, the Euphos 

 glass was replaced by the light crown glass as before with the result 

 of showing that substantially one half, 49.9 %, of the total ultra violet 



