80 De. EAivr on the Detection of Arsenic. 



gently in a small tube, yielded a slight but distinct sublimate, most obvious 

 on a black ground, and which, with a magnifying power of ten to twenty 

 diameters, was found to consist of crystals with triangular facettcs, and 

 which, when dissolved in water, yielded orpiment and the red arseniate of 

 silver, when treated with the appropriate reagents. 



This I believe to be as great a degree of delicacy as has actually been 

 obtained by the more tedious and troublesome process of Marsh ; and is 

 more than sufficient for every practical purpose. 



6. When investigating the delicacy of Eeiusch's process, I prepared 

 quantities of very dilute solutions of arsenious acid, varying in strength 

 from 70,0 cTo *^ Toolooo' ^""^ ^^P*' t^i^se solutions in readiness for the experi- 

 ments which I had planned. When first tried with copper and muriatic acid, 

 they gave results entirely conformable to those already stated. A portion 

 of any of these solutions, containing one thousandth of a grain of arsenious 

 acid, when diluted, so that the fluid amounted to a million times the 

 weight of the arsenic, gave a distinct and rapid deposit on the copper ; 

 but afterwards I could obtain no deposit from larger quantities of arsenious 

 acid, though in a more concentrated state. As an example — a portion of 

 solution containing g^^ of a grain of arsenious acid in sixty grains of 

 water, and consequently with a dilution of one in twelve thousand, gave no 

 deposit whatever when boiled in the ordinary way with copper and muriatic 

 acid for upwards of fifteen minutes. In this case the surface of the 

 copper was only |- of a square inch, and therefore could not interfere with 

 the result by its too great extent. 



I was perplexed with the apparent inconsistency of these results with 

 those previously detailed, and began to suspect that I had been misled in 

 my first estimate of the extreme delicacy of Reinsch's process. But after 

 repeated trials I found the difference to depend on the length of time that 

 the solution is kept. Very dilute solutionis of arsenious acid hecome 

 gradually less and less sensitive to Beinsch's process, so that after several 

 weeks no deposit can be obtained on copper from solutions containing 

 arsenic in the proportion of one in Jijly thousand, or even one in twenty 

 thousand. I have recently found, however, that the addition of a small 

 quantity of any animal matter, such asmilk, effectually preventsthis change. 



7. I was thus led to examine whether these dilute solutions underwent 

 any appreciable change in their chemical properties, and found that with 

 nitrate of silver they gave a white cloud — when concentrated by evapora- 

 tion to a small bulk, the residual fluid strongly reddened litmus, and when 

 evaporated to dryness, left a white stain, which did not sublime at a low 

 red heat. This stain redissolved in a few drops of water, forming a 

 solution which still strongly reddened litmus, and which, on the addition 

 of a strong solution of nitrate of silver, gave a brick red precipitate. 



These experiments clearly indicate the conversion of arsenious acid 

 into arsenic acid. 



8. In all watery solutions of arsenious acid this change appears to 

 take place to a certain extent, if the solution is kept for a considerable 



