OP ARTS AND SCIENCES. 27 



and has been in use in that hiboratory and others with excellent suc- 

 cess. The proof of the availability of the method was undertaken by 

 me, but, owing to numerous interruptions, the completion of the ana- 

 lytical work has been delayed until now. 



The method consists, briefly, after getting the arsenic from a 

 measured amount of paper or fabric into solution, in the comparison 

 of the mirror obtained from an aliquot part of the solution with a 

 series of standard mirrors obtained from known amounts of arsenious 

 oxide. No method founded on exactly this principle has ever been 

 fully described, though Otto * gives cuts of mirrors obtained from 

 known amounts of arsenious oxide, with which some analysts may 

 have compared their mirrors. Selmi,t in 1880, states that he is 

 able to approximate to fractions of a milligram by comparing the 

 mirrors with those obtained from the following amounts : one twen- 

 tieth, one fiftieth, one hundredth, and one two-hundredth part of a 

 milligram. Thoms, as stated above, compares his " traces " with 

 a mirror obtained from one tenth of a milligram, and says that the 

 mirror can be kept any length of time as a comparison standard. 

 Blyth $ also suggests a comparison of mirrors. 



I give in detail the method as I have used it in the analysis of wall 

 paper, making references to the analytical and experimental work 

 which follows. 



The measurement of the paper is governed by (a) the quantity 

 of arsenic present, which may sometimes be judged by the color, 

 or by the rough test of the odor from the burning paper, and 

 (h) by the character of the paper ; i. e. whether a plain color, a 

 small or large figure. I have used 25 sq. cm., 50 sq. cm., and usually 

 100 sq. cm. As patterns for cutting, thin plates of glass § may be 

 used, on which are marked the dimensions. The advantage of the 

 glass is, that the figure of the paper may be seen while the paper is 

 being cut, and also, that, by washing or wiping the glass after each 

 cutting, any danger of contamination by adhering particles from a 

 previous arsenical paper may be avoided. I have used but one plate 

 for 25 sq. cm. (5x5); for 50 sq. cm., three (5 X 10, 4 X 12.5, and 

 2 X 25) ; and for 100 sq. cm., five (10 X 10, 5 X 20, 4 x 25, 8 X 12,5, 

 and 2 X 50) ; such a number allowing the variety in cutting that dif- 

 ferent papers necessitate. 



* Graham-Otto-Michaelis, Lehrbuch, II. 2, 520. 

 t Gazz. Cliim. Ital., X. 435. 



X Poisons, tlieir Effect and Detection, 1884, p. 634. 

 § Dr. Charles Harrington. 



