110 Dr. 0. J. Lodge on a Method of measuring the 



positions of the more important lines of the elements whose 

 spectra are easily obtained with the aid of the Bunsen flame. 

 The symbol of the element to which a particular line or band 

 belongs is legibly written under the particular point of the 

 scale, and connected by a line with the point in question. 

 Identification of the bright lines observed in the spectrum of 

 an unknown compound is thus greatly facilitated. 



I may be permitted to add that the measuring-apparatus 

 described has been fitted to the spectroscope used in the College 

 Laboratory by Messrs. Yeates and Son of this city, whom I 

 have to thank for the care and skill with which they carried 

 out the details of construction. 



XVI. On a Method of measuring the Absolute Thermal Con- 

 ductivity of Crystals and other rare Substances. — Part I. By 

 Oliver J. Lodge, D.Sc* 



1. "YXTHEN only a small portion of a substance is obtain- 

 ▼ * able on which to experiment, the measure of con- 

 ductivity by any dia-calorimetric method becomes difficult ; and 

 accordingly observers have contented themselves, in the case of 

 the rarer crystalline bodies, with comparing their conductivities 

 in different directions by Senarmonfr's or some similar method. 

 If the substance is sufficiently plentiful to be obtained in slabs 

 (like rocks), then some modification of Fourier's "thermo- 

 metre de contact" will give its conductivity, though there 

 are many objections to the use of this instrument. 



But there is another method of Fourier's, applicable only to long 

 rods, put in practice by Biot, Despretz, Forbes, and recently by 

 Wiedemann and Franz (commonly known as Forbes's method), 

 which it seems possible to modify so as to make it applicable 

 to short rods or even slicesf. This well-known method consists 

 in observing the permanent curve of temperature along a cy- 

 lindrical rod of the given material, one end of which is heated 



* Communicated by the Physical Society. 



t The method occurred to me when thinking how best to measure the 

 conductivity of tourmaline in opposite directions along the axis, a subject 

 which I was considering in conjunction with Mr. S. P. Thompson of 

 Bristol ; for we had reason to think that tourmaline and all other pyro- 

 electric crystals must necessarily possess a unilateral conductivity along 

 their axis both for heat and electricity ; and this supposition has been 

 partially confirmed, in the case of heat, by some preliminary experiments 

 of Mr. Thompson's last summer on a very small crystal. No further con- 

 firmation or modification of the experiment, however, has yet been pos- 

 sible, owing to the scarcity of the crystal and the difficulty of obtaining 

 a large slice ; but this difficulty has now been removed by the kindness 

 of Professor Nevil Story Maskelyne. 



