W. S. Yeates — Pseudomorphs of Native Copper. 405 



with, rise of temperature. The change is, however, so small 

 as to be quite negligible within the range of temperature to 

 which a normal element is subjected in practice. Lord Ray- 

 leigh's cells show a change in the temperature-coefficient 

 directly the reverse of the above ; that is, the coefficient in- 

 creases by a very appreciable quantity with rise of tempera- 

 ture. For his No. [36] the coefficient ranged from '000556 at 

 0° C. to -00101 at 25° C, if his equation holds true for the 

 higher temperature. 



In making comparisons of E. M. F. I have used Lord Ray- 

 leigh's method, slightly modified, by means of which a difference 

 of one ten-thousandth part is observed directly and with the 

 greatest ease. In fact a difference of half that amount is 

 easily measured. A comparison of half a dozen cells can be 

 made in as many minutes without difficulty. 



As to polarization, these cells show none with external resist- 

 ance greater than 30,000 ohms. At 30,000 ohms the polariza- 

 tion is just discernible ; and with 10,000 ohms it amounts to 

 only one ten-thousandth part in five minutes. This fall in E. M. 

 F. is less than the accidental differences between different 'cells 

 in general, and much smaller than the almost unavoidable errors 

 due to ignorance of the real temperature of the cell. If the 

 cell is not closed on less than 10,000 ohms resistance, and only 

 for a few minutes, the polarization may be entirely neglected. 



As indicating the uniformity attained, the following relative 

 values of the E. M. F. of six cells, only four days old, may be 

 given : 9048, 9049, 9049, 9048, 9046, 9043. The last one was 

 still approaching the others when last observed. Six cells of 

 later construction gave the following relative values when less 

 than two days old: 9182, 9182, 9182-5, 9182, 9182,9182-5. 

 The two sets of numbers do not represent at all the relative 

 values of one set as compared with the other. 



It will be seen from the table that Nos. 112 and 113 never 

 differ by more than one part in ten thousand at the same tem- 

 perature. 



Art. LI. — Pseudomorphs of Native Copper after Azurite, 

 from Grant County, New Mexico ; by W. S. Yeates. 



During the month of April of this year, Mr. J. A. Lucas 

 of Silver City, Grant County, New Mexico, sent to the U. S. 

 National Museum a specimen with this brief note : — " From 

 the ' Copper Glance' and 'Potosi' copper mines, Grant Co., 

 New Mexico. This ore is found in all imaginable shapes and 

 sizes from 1 oz. to 70 lbs." The specimen was referred to the 



