J292 



Lord Elphinstone and C. W. Vincent. [Nov. 20. 



degree, the power of absorbing the ultra-violet rays of the spectrum, 

 though they are inferior in this respect to benzene and its derivatives, 

 to which class of bodies they are so closely allied. 



2. Terpenes with the composition C 15 Ho 4 have a greatly increased 

 absorptive power for the more refrangible rays, that is to say, they 

 withstand dilution to a greater extent the greater the number of carbon 

 atoms in the molecule. 



3. Neither the terpenes themselves nor the oxidised or hydrated 

 derivatives occasion absorption bands under any circumstances when 

 pure, but always transmit continuous spectra. 



4. Isomeric terpenes transmit spectra which generally differ from 

 one another in length, or show variations on dilution. 



5. The process of diluting with alcohol enables the presence of 

 bodies of the aromatic series to be detected in essential oils, and even 

 in some cases the amount of these substances present may be estimated. 



Several diagrams in illustration of the kind of absorption exerted 

 by the different substances are presented with the complete paper. 



III. " Preliminary Note on Magnetic Circuits in Dynamo- and 

 Magoeto-Electric Machines." By Lord Elphinstone and 

 Charles W. Vincent, F.R.S.E./f.C.S., F.I.C. Received 

 July 26, 1879. 



The experiments which form the subject of the present note were 

 made in connexion with an investigation as to the best form for the 

 construction of a dynamo-electric machine, intended to furnish cur- 

 rents of high intensity in great quantity. The principle deduced 

 applies equally to magneto- electric machines. 



The source of power in all dynamo- electric machines being electro- 

 magnets whose cores are already slightly magnetic, it appeared to us 

 necessary to consider the conditions under which the initial force of 

 such machines is best obtained. 



For this purpose we made use of a \J electro-magnet having a core 

 of soft iron 2 inches in diameter and 36 inches long. The arms of the 

 U were 4 inches apart. The exciting helices were two sheet copper 

 reels, 12 inches long, fitting closely upon the uprights of the [J , but 

 readily removable. Each of these reels was coiled with 200 yards of 

 No. 14 double covered copper wire. 



Two cores of soft iron, of the same diameter, and each 12J inches 

 long, and which could be magnetised by the same helices, were also 

 employed. 



The principal armature was of soft iron, 8 inches in length, by 

 .2 inches in width, and 1 inch thickness, rounded at the ends. Its face 



