228 Royal Society : — 



101, 101, and through the faces 1 1 0, 1 1 0, the crystal being 

 immersed in water contained in a vessel bounded by plates of glass 

 parallel to the plane bisecting the dihedral angle formed by the 

 refracting faces in each case. From these observations it appears 

 that for a ray in the plane 10 0, and polarized in that plane, the 

 index of refraction is about 1*837 ; for a ray in the plane 10, and 

 polarized in that plane, the index of refraction is between 1*545 and 

 1*546 ; and for a ray in the plane 1, and polarized in that plane, 

 the index of refraction is about 1*545. Hence the optic axes are in 

 the plane 10, and they make with each other a small angle which 

 is bisected by the line [10 0]. 



A crystal having two opposite faces of the form 110 much larger 

 than the two remaining faces, being immersed in oil for which 

 p=V4793, and placed in a polarizing apparatus, the rings sur- 

 rounding the optic axes were seen through the large faces of the 

 form 110. The angle included between the directions of the optic 

 axes within the oil was about 4°. 



"On the Synthesis of Leucic Acid." By Dr. Edward Frank- 

 land, F.R.S. 



" On the Artificial Production of Fibrine from Albumen." By 

 Alfred Hutchinson Smee, Jun. 



"Note on the Spectrum of Thallium ." By Professor William 

 Allen Miller, M.D., LL.D., Treasurer and V.P.K.S. 



My friend Mr. Crookes, the discoverer of the new metal thallium*, 

 having kindly put into my hands a small quantity of the metal, which 

 he believes to be chemically pure, I have been enabled to make some 

 experiments upon its spectrum, the results of which may not be with- 

 out interest to the members of the Royal Society. 



Thallium, as is well known, when examined in the usual way by 

 the spectroscope, yields a spectrum of remarkable simplicity, furnish- 

 ing a single intense green line, the occurrence of which, as is familiar 

 to chemists, led Mr. Crookes to the discovery of the metal, and sug- 

 gested to him the name by which it is known. In order to try the 

 effect of a progressively increasing temperature upon the spectrum 

 furnished by the metal and its compounds, the following experiments 

 were made. 



Portions of metallic thallium, as well as of an alloy formed by 

 fusing a bead of thallium upon the end of a platinum wire, and por- 

 tions of the sulphate of the metal were introduced successively, 

 first, into the flame of burning hydrogen, and then into the oxy hy- 

 drogen jet, and were in each case viewed by the spectroscope. As 

 the temperature increased in intensity, the brilliancy of the thallium 

 green line increased also, but no new lines made their appearance. 



* It has been made the subject of question abroad, whether Mr. Crookes or M. 

 Lamy was the first to recognize the metallic nature of thallium, and thus to 

 dispute the claim of Mr. Crookes to the full credit due to him for his investigation 

 (with only about twenty grains of the element) of its leading characters where no 

 previous clue existed to guide him. It may be sufficient to state in answer to this 

 suggestion, that Mr. Crookes had exhibited it at the International Exhibition, and 

 marked as metallic his scanty store, though in the form of a precipitate, in the 

 beginning of May, unquestionably before M. Lamy had published anything re- 

 lating to thallium. 



