282 Mr. Marsh on distinguishing Arsenic from Antimony. 



of the scale, because there the cross striae lie much closer to 

 each other. But if they are examined with more powerful 

 instruments, and the forms adjusted so as to see the rising and 

 sloping portion of the cross stria;, then instead of the diagonal 

 line appearing, the ordinary longitudinal lines will appear 

 *' like short hairs or spines in a diagonal direction." In this 

 way they appeared to Mr. Pritchard "through his best instru- 

 ments." The varieties of appearance seem to me to arise from the 

 instrument commanding at different times the different heights 

 of an uneven surface, and the difference depends upon the di- 

 stinct vision of the hill, the valley, or the intermediate slope, 

 in a distance which between each longitudinal rib is only the 

 20,000dth of an inch. The cross striae are about the 60,000dth 

 of an inch apart, and near the end of the scale still less. 

 I remain, Gentlemen, yours, &c. 

 Burton Latimer, Aug. 31, 1839. Edward CraIG. 



Description of the Figures. [Plate I.) 



1. A section of the scales, showing the wavy nature of the 



ribbed surface. 



2. A portion of the scale, showing only the longitudinal 



lines, and torn along one of the fibres. 



3. A portion exhibiting the structure of the film, with the 



longitudinal and cross striae. 



4. A portion in which the dark longitudinal striae are 



thrown out of the focus, and only the cross striae 

 shown. 



5. A portion in which the delusive appearance of the dia- 



gonal lines is attempted to be imitated. 



6. A portion in which the focus of the lens is so adjusted 



as to show only a portion of the cross striae on the 

 rising slope of the furrow. 



XXXVIII. On a new Method of distinguishing Arsenic from 

 Antimony, in cases of suspected poisoning by the former 

 substance. By Mr. J. Marsh.* 

 T N testing for arsenic in cases of poisoning by that substance, 

 -* it has always been desirable to render the process as simple 

 as possible, and thereby divest the mind of any ambiguity on 

 the subject. It was with this view that I submitted to the Society 

 of Arts, &c. in the year 1836, my process by hydrogen, a 

 process that I then fondly hoped would have removed all 

 difficulties; but a communication from my friend Mr. Lewis 



* Communicated by the Author. 



