420 Dr. Genth's Conhnhutions to Metallurgy. 



a transmitted beam can be made by an opake screen, and so a 

 beam of plane-polarized light can be obtained. When the com- 

 mon double-image rhomb is placed in the same manner before 

 the eye, no boundaries of total reflexion are seen. 



I have tried the rhomb above named, with an- between the 

 component prisms, in various experiments where Nicol's rhomb 

 would generally be used, and found it succeed ; amongst others, 

 I have used it as polarizer and analyser in the microscope : 

 biit I have not found any advantages to recommend it in 

 preference to NicoFs rhomb, except a saving of the material, 

 calc-spar ; and the smallness of the angle gives some disadvan- 

 tages. The only cases in which it would have a decided advan- 

 tage, are those in which it was proposed* to be used where a 

 large breadth of pencil was required with small angular diver- 

 gence or convergence ; and then convergence in the transmitted 

 beam may be produced by a convex lens. In this way an ana- 

 lyser may be formed so as to throw images of selenitc designs 

 upon a screen for exhibition to a number of persons better than 

 by any other method for brightness and distinctness ; and there 

 may be found other peculiar cases where it would have an ad- 

 vantage over other rhombs. 



XL VIII. Contributions to Metallurgy. — No. I. 

 By Frederick A. Genth, Ph.D.f. 



THE difference in the appearance of a lot of Chinese coins 

 which were lately offered for sale in this city by a native 

 of the Celestial Empire, and the little knowledge which we have 

 on the subject of Chinese metallurgy, suggested the idea that, 

 by a careful examination of the different kinds, some new and 

 serviceable alloys might be added to those already used in the 

 arts, or, at least, that by such an investigation some informa- 

 tion might be obtained of the Chinese ores, and the manner 

 of their reduction. With the exception of three or four alloys, 

 which have been repeatedly analysed, we know hardly anything 

 of the metallic compositions used by the Chinese. I have en- 

 deavoured to learn from the works written on China, how their 

 alloys were manufactured, what kind of ores they use for that 

 purpose, whether they prepare their alloys directly from the ores, 

 or by mixing the metals in the proper proportions, &c. Unfor- 

 tunately, I have been able to consult bat few books ; and these 

 have frequently given statements which, on account of the want 

 of familiarity with metallurgy of their authors, are unquestion- 



* Potter's ' Physical Optics,' p. 31. 



t From the Joiunal of the Franklin Institute, S. 3. vol. xxxvi. p. 261. 



