332 



On Tests for Arsenic. 



But if the inside diameter of the tube be larger at one 

 end than at the other, the corrections cannot be found by 

 the nrecediniz; rule; but by the following method they may 

 be obtained/with the same precision as before. 



Fill about five or six inches of the sealed end of the 

 tube with quicksilver; and after having determined the 

 altitude of the surface of the mercury in the cistern, pour 

 that which is in the tube into it, and measure how high 

 the surface of the fluid has been raised. Then divide this 

 rise in the cistern by the number of inches of mercury 

 poured into it, from the tube, and the quotient will be the 

 correction for one inch rise of the mercury above the zero. 



Suppose the number of inches of quicksilver in the tube 

 were five, and this quantity raised the surface of the fluid 

 in the cistern 3-lOOdths of an inch, then the correction 

 would be 1-lOOdth of an inch, for every inch rise of the 

 mercury above the zero ; from whence all the other cor- 

 rections may be found. 



If the numbers found by these rules were engraved upon 

 the plate of the instrument, in some form like Table 2, or 

 written upon a slip of paper and pasted upon the frame, 

 the altitude of the quicksilver in the tube might then be 

 corrected by inspection, and perhaps to as great a degree 

 of precision as by any other method. 

 I am, sir, 



Your obedient servant, 



Lynn, Nov. 13,1812. Ez. WaLKETJ. 



LVIII. On Tests for Jrsenic. By P. M, Roget, M.D. 



To Mr. Tilloch. 



Sir, It is with reluctance that I again trouble you on a 

 subjfct in which tlie public is so little interested, as the 

 controversy respecting the priority of Dr. Marcet or Mr. 



Ilume> 



