226 Notices respecting New Books. 



which can be used as reference-lines, by measurement from which 

 the wave-lengths of the metal-lines can be determined. 

 • Thus in all the spark-spectra the lines of air are present in large 

 number throughout the spectrum. 



The possessor of this book will find it an advantage to mark the 

 wave-lengths of the lines in the air-spectrum, which can best be 

 done on the enlarged photograph of the air-spectrum given in the 

 extra plate at the end of the volume. 



The line at the red end (marked " spark " in the scale on Plate I.) 



5005 ] 

 is the nitrogen double line r^n [■ . The next group in the large 



photograph is a group of nine lines, the third of which (the brightest) 

 has the wave-length 4630, and the finer ones 4642, 4640, 4621, 

 4613, 4607, 4601, 4596, 4591. In the second photograph on 

 Plate I. there are between these a group of three lines, 4803, 4788, 

 4779. The next conspicuous line in the large photograph is 4447, 

 and then (with a faint band between) 4416. Then comes a broad 

 mass of lines occupying some inch or so in length. The least-re- 

 frangible bright line of this group (not the fine one) is 4348. The 

 centre of the broad band is 4230, then a very close pair of lines, 

 4190, 4184. Commencing now at the blue end of the spectrum, 

 there is a solitary bright line 3995, then a hazy band the brightest 

 part of which measures 4038, and then, easily recognizable in all 

 the photographs, a double line (really triple) followed by a some- 

 what wider double, and then three equidistant lines with a still 

 wider interval. These read : — 



4069 1 , inft 4123 



4074 J li(U and 4137 



4076 * 1U * 4149 



This last is followed by the line 4155 ; and between this and the 

 broad band 4230 is a fine double * -. q. 



These air-lines are present in nearly all the spark-spectra (scarcely 

 recognizable in the spectra of Barium and Strontium). The metal- 

 lines may commonly be distinguished by their different character — 

 for example, the Bismuth-lines 4392, 4259, 4560, and 4722, in the 

 second photograph of Plate III., and the Lead-lines 4058, 4246, 

 and 4385, in the first spectrum of Plate XI. The substances to 

 which the lines are due must be decided by internal evidence ; for 

 there is no evidence given of the chemical purity of the metal em- 

 ployed, and there is frequent evidence of impurity : for example, 

 the Buthenium-spectruni is chiefly due to iron. 



The value of the book would be much increased by a systematic 

 identification of the lines — a work which the author does not seem 

 to have very carefully attempted; at least he remarks that the 

 Cadmium spark-spectrum is " clear of air-lines," whereas the fact 

 is that out of about 21 lines all but 2 are due to air. By such a 

 systematic investigation, the spectra of many elements which have 

 been pretty carefully investigated might be extended considerably 

 towards the blue end with a certain amount of precision. 



