290 Prof. W. N. Hartley and A. K. Huntington. [Nov. 20. 



Lastly, the authors say : — 



"We have again pleasure in thanking Professor Stokes for his 

 much- valued advice during the course of our investigations. To our 

 assistant, Mr. Fram, we are indebted for his able co-operation, and 

 we have to thank Mr. H. Reynolds for his aid and skill in taking 

 photographs." 



II. " Researches on the Action of Organic Substances on the 

 Ultra- Violet Rays of the Spectrum. Part III. On Exami- 

 nation of Essential Oils." By W. N. Hartley, F.R.S.E., 

 &c, Professor of Chemistry in the Royal College of Science 

 for Ireland, Dublin, and A. K. HUNTINGTON, F.I.C., F.C.S., 

 Associate of the Royal School of Mines. Received July 22, 

 1879. 



(Abstract.) 



Much chemical and physical research by various investigators has 

 been devoted to the class of bodies known as Essential Oils, as, for in- 

 stance, the work of Dr. J. H. Gladstone ("Journal of the Chemical 

 Society," vol. xviii, p. 1 ; vol. xxiii, p. 147 ; vol. xxv, p. 1) ; of Dr. C. 

 R. A. Wright ("Journal of the Chemical Society," vol. xxvi, pp. 549 

 and 686; vol. xxvii, pp. 1, 317, and 619, Isomeric terpenes and their 

 derivatives) ; and of Dr. W. A. Tilden (he. cit., vol. xxviii, pp. 514 and 

 1258), as well as of many others. 



The new method of research employed by us and described in a 

 paper about to be published in the " Philosophical Transactions," has 

 been applied to the examination of these substances. We have to 

 acknowledge the kindness with which several gentlemen have supplied 

 us with samples of essential oils, namely, Dr. Gladstone, Mr. Parries, 

 of the firm of Burgoyne, Burbidges, Cyriax, and Farries, Dr. Sep- 

 timus Piesse, and Dr. W. A. Tilden. 



As in our previous experiments (Abstracts of Parts I and II, 

 " Proc. Roy. Soc," No. 192, 1879), photographs w^ere taken of the 

 spectrum transmitted by the undiluted liquid, and then of that trans- 

 mitted by the liquid in various states of dilution, the dilutions, ranging 

 in some cases from 1 in 50 to 1 in 500,000 volumes of alcohol. 



The following is a list of substances examined, classified according 

 to the optical properties they were found to possess. 



Oils and hydrocarbons transmitting continuous spectra. 



Australene, from oil of turpentine. Calamus. 



Birch bark. Citron. 



Cajputene dihydrate. Citronella. 



Carraway hydrocarbon (No. 2). Cedar wood. 



