308 REPORT— 1880. 



from the results were in consequence necessarily limited. Apparatus 

 employed: — a Ruhmkorff's coil, a magneto-electric machine, and four 

 Leyden jars were so arranged as to give a spark 15 centimetres long. 

 The spectroscope with a fluorescent eyepiece was employed in this and 

 subsequent investigations of a similar nature. 



Transparency of Quartz, Sfc. 



The observation by Prof. Stokes, that quartz when beyond a certain 

 thickness does not transmit the extreme rays was confirmed, a gradual 

 absorption being found to take place as the thickness increased ; he also 

 found that a prism of Iceland-spa absorbs the very extreme rays. From 

 a comparison of the transparency of quartz and water, the conclusion was 

 arrived at that ' the coefficient of extinction for the rays of the refrangi- 

 bility of the ray 32 is feebler for quartz than for water ; but for the rays 

 27-31 this coefficient is stronger for quartz.' It was ascertained that the 

 transparency of solutions filtered through paper was not impaired pro- 

 vided the paper had been previously washed with water containing a little 

 hydrochloric acid, and then with pure water. 



If the thickness of the layer through which the rays have to pass be 

 increased, the very refrangible rays are more and more intercepted ; but 

 the rate of diminution varies much with diSerent substances. Chromates 

 and nitrates were observed to cause absorption bands in the ultra-violet 

 region. The sulphates of didymium and cerium, and commercial ammonia 

 were also found to occasion absorption bands, that in the case of ammonia 

 being, however, due to some impurity ordinarily present in ammonia de- 

 rived from gas- liquor. 



MM. J. L. Soret and A. A.. Rilliet have since examined the ultra- 

 violet absorption-spectra of the ethereal nitrates and nitrites with the view 

 to ascertain whether these substances behave in the same way as metallic 

 nitrates and nitrites.' 



The nitrates of ethyl, butyl, and amyl were found to absorb energeti- 

 cally the ultra-violet rays ; but they did not produce the characteristic 

 absorption-bands observable in the case of the metallic nitrates. The 

 ethereal nitrates are, however, more transparent than the metallic niti'ates 

 for the rays 12-14, but less so for 17-20, being again more transparent 

 for the extreme rays. The vapours of the ethereal nitrates exhibit con- 

 siderable absorptive power even at the ordinary temperatures. 



Nitrites of amyl and ethyl absorb energetically the ultra-violet rays. 

 Six bands, about equidistant, are apparent between H and R. The vapours 

 give the same bands. The alkaline nitrites, although veiy absorbent for 

 this part of the spectrum, do not give the same absorption-bands. 



An examination of the absorption-spectra of the bases in gadolinite by 

 means of the solar rays, an Iceland spa prism being used, has led ]\I. 

 Soret to the conclusion that some of the bands in the ultra-violet region 

 are not due to yttrium, erbium, or terbium, but to the new base dis- 

 covered by Delafontaiue.'^ In a subsequent communication on the 

 absorption-spectra of didymium and some other substances obtained from 

 samarskite,^ a comparison is given between a chloride of didymium from 

 samarskite, and one obtained from a difierent source. The former ex- 

 hibited difierences, supposed to be due to the presence of the new earth 

 in small quantity. Further on in the same volume (page 521) we find 



' Comjjtes Benliif, t. Ixxxix. p. 747. ^ Ibid, t, Ixxxvi. p. 1062. 



' Zoc. cit. Ixxviii. p. 422. 



