128 REPORT— 1882. 



Erhard' bas examined the absorption spectra of some salts in which 

 chromium plays the part of a base. It may be said as a general rule that 

 these salts absorb the yellow and yellowish-greeu and also the violet end 

 of the spectrum, transmitting the blue ; the exact position of the maxi- 

 mum of absorption, however, and the intensity of the absorption band, 

 varies considerably with different salts, and even for the same salt with 

 different temperatures, and the results are complicated by the fact that 

 heating the salts produces a permanent alteration in the absorption. The 

 insoluble chloride of chromium shows a behaviour differing from that of 

 the other chromium salts. It transmits the yellow and more of the violet 

 than the other salts. Some of the solid crystals of various chromium 

 salts show fine absorption bands in the red which can also be traced in 

 some of the solutions. There is therefore a general resemblance in the 

 absorption of different chromium salts, but no identity. 



Dr. Gladstone has also examined the eifect of chlorine, bromine, 

 and iodine when combined with different metals. The bromides of gold, 

 platinum, palladium, and potassium give a spectrum which is identical 

 with that of bromine water ; the same applies to a concentrated solution 

 of the bromide of copper, which in addition shows the red absorption 

 characteristic of copper. A dilute solution of bromide of copper shows, 

 however, no absorption which can be traced to the bromine. Similar 

 results were obtained with the chlorides and iodides. In pointing out 

 that it is generally though not universally true that a base or an acid 

 retains its absorptive properties in different combinations, Dr. Gladstone 

 draws attention to the remarkable exception of ferric ferrocyanide, which 

 when dissolved in oxalic acid transmits blue rays in great abundance, 

 though the same rays are generally absorbed both by ferrocyanides and 

 by ferric salts. 



Nitric acid and the nitrates of transparent bases such as potassium, 

 sodium, and ammonia show spectra, according to Soret, which are not 

 only qualitatively but also quantitatively identical ; that is to say, a 

 given quantity of nitric acid in solution gives a characteristic absorption 

 band of exactly the same width and darkness whether by itself alone 

 or combined with a transparent base. It also shows a continuous absorp- 

 tion at the most refrangible side, beginning with each of the mentioned 

 salts at exactly the same point. The ethereal nitrates,^ however, give 

 different results. 



Messrs. Hartley and Huntington have by photographic methods 

 examined the absorption spectra of a great number of organic compounds. 

 As their researches have already been referred to at length in these 

 reports,' by one of the authors, we need at present only mention one or 

 two of the results which most interest us from our present point of view. 

 The normal alcohols were found to be transparent for the ultra-violet 

 rays, the normal fatty acids less so. In both cases an increased number 

 of carbon atoms increases the absorption at the most refrangible end. 

 The fact that benzene and its derivatives are remarkable for their power- 

 ful absorption of the most refrangible rays, and for characteristic ab- 

 sorption bands appearing on dilution, led Professor Hartley to a more 

 extended examination of some of the more complicated organic substances. 

 He came to the conclusion that definite absorption bands are only pro- 



' Tnavpiral Dissertation, Freiburg (without date). 

 [ » Brit. Ass. Be]). 1880, p. 55. ' C. B. Ixxxix. p. 747. 



