On the Spectra of some of the Chemical Elements. 541 



that at its momentary limit, at the point on which it has jnst arrived, 

 the divellent force on the molecules there situated is counteracted 

 only by half the cohesive force which acted when there was no crack, 

 viz., the cohesion of the uncracked portion alone. But if a crack 

 anywhere produced be stopped from running by encountering a solid 

 particle of greater cohesive force, fracture will no longer be determined 

 by the accidental deficiency of cohesion at some weak point, but by 

 the average resistance of the whole cementing mass. 



It would, I think, be interesting to determine, on the more trans- 

 parent portions of glacier ice, by the simple and easily applied test of 

 polarized light, whether a definite crystalline structure prevail in its 

 interior, and if so, in what direction the axis lies in relation to the 

 lines of fissure in the crevasses. Nor is there any reason why the 

 idea above thrown out respecting the mutual modification of structure 

 of two masses cemented by regulation, at or near their plane of junc- 

 tion, should not be subjected to a similar test. 



December 10. — Major-General Sabine, President, in the Chair. 



The following communication was read : — 



"On the Spectra of some of the Chemical Elements.'' By Wil- 

 liam Huggins, Esq., F.R.A.S. 



The author has been engaged for some time in association with 

 Prof. W. A. Miller in observing the spectra of the fixed stars. For 

 the purpose of comparing the spectra of these with the spectra of 

 the terrestrial elements, no maps of the latter were found that were 

 conveniently available. KirchnofFs maps and tables, besides their 

 partial incompleteness, were not suited for night work when the sun 

 could not be simultaneously observed. 



The author adopts the lines of the spectrum *of common air as the 

 fiducial points of a standard scale to which the spectra of the elements 

 are referred. The air-spectrum has the advantage of being always 

 visible with the spectra of the metals without increased complication 

 of apparatus. 



The observations were made with a spectroscope of six prisms of 

 heavy glass of large size. The total deviation of the light with this 

 train of prisms is for the 1) ray about 198°. The telescope and 

 the collimator have both an aperture of \'7 inch. The focal length 

 of the telescope is 16*5 inches. The measures were partly taken from 

 the readings of a finely divided arc of brass, which the arm carrying 

 the telescope traverses, and partly from the readings of a wire micro- 

 meter attached to the eye-end of the telescope. The scale of mea- 

 surement adopted gives five divisions for the interval between the 

 components of the double line D. The excellent performance of 

 this instrument is shown by the great distinctness of the finer lines 

 of the solar spectrum. All those mapped by KirchhorT are seen, 

 and many others in addition to these. 



The spark of an induction coil was employed, into the secondary 

 circuit of which a battery of nine Ley den jars was introduced. The 

 Leyden jars are arranged in three batteries of three jars each, and the 

 batteries connected in series. 



