1890.] Tlie Spectroscopic Properties of Dust. 437 



(ii) The use of a standard coil with a single layer of wire, the 

 coefficient of mutual induction of the coil and circumference of 

 the disc being calculated by a formula obtained by the direct 

 integration of the expression 



JJ 



ds ds n 

 cos e 



for a circle and coaxial helix. 



(iii) The use of a new form of contact brush at the disc circum- 

 ference, which procures greatly increased steadiness in the 

 galvanometer needle. 



The brush consists of a single wire of phosphor bronze, perforated 

 by a channel through which a continuous flow of mercury is 

 maintained from a cistern of adjustable height. 



Incidentally, a description is given of an accurate method of measur- 

 ing the vibration frequency of a standard tuning-fork by means of a 

 Bain's electrochemical telegraph receiver. 



In conclusion, suggestions are made towards a new determination 

 of the ohm that shall be final for the practical purposes of the 

 electrical engineer. 



II. " The Spectroscopic Properties of Dust." By G. D. 

 LIVEING, M.A., F.R.S., Professor of Chemistry, and J. 

 DEWAR, M.A., F.R.S., Jacksonian Professor, University of 

 Cambridge. Received August 16, 1890. 



The suggestion that the auroral spectrum, the principal ray in the 

 spectrum of nebulae, and other rays of unknown origin, might be due 

 to meteoric dust induced us to investigate the problem whether solid 

 particles of sufficient minuteness would act like gaseous molecules in 

 an electric discharge and become luminous with their characteristic 

 special radiation. The dust we employed was that thrown off from 

 the surface of various electrodes by a disruptive discharge, and it 

 was carried forward into the tube of observation by a more or less 

 rapid current of air or other gas. The arrangement will be best 

 understood from the annexed diagram, which represents a section of 

 the glass vessel which was the principal part of the apparatus. A 

 represents a bulb in which were the electrodes a, a to give the dust, 

 connected by a widish tube d with the tube for observation B. The 

 end E was blown clear, so that the narrow part of B could be ob- 

 served end-on. The electrodes e, e were of platinum. The tube g, 

 passing from A to the supply of gas, was fitted with a glass stopcock 



