ARSENIOUS ACID 



yellow precipitate of tribasic phosphate of silver, which exactly resembles the 

 areenite. The phosphate is, however, the more soluble in ammonia, and when heated 

 gives no volatile product; while the arsenite is decomposed into white arsenic and 

 oxygen, leaving metallic silver behind. 



4. Ammoniacal Sulphate of Copper produces a fine apple-preen precipitate of 

 arsenito of copper, which is dissolved in an excess of either acid or ammonia. It 

 is, however, uncertain, unless the precipitate bo dried and reduced. 



6. The Reduction Test. Any portion of the suspected matter, being dried, is mixed 

 with equal parts of cyanide of potassium and carbonate of potash, both dry. This 

 mixture is to be introduced into a tube terminating in a bulb, to which heat is 

 applied, when metallic arsenic sublimes. 



6. Marsh's Test. This is one of the most delicate and useful of tests for this poison, 

 and when performed with due care there is little liability to error. The liquid con- 

 tents of the stomach, or any solution obtained by boiling the contents, is freed as 



much as possible from animal 



^J^j^gji^ , f , --S3 matter by any of the well- 

 known methods for doing so. 

 This fluid is then rendered 

 moderately acid by sulphuric 

 acid, and introduced into a 

 bottle properly arranged. 



Fig. 83 is the best form 

 for Marsh's apparatus: a 

 is a bottle capable of holding 

 half, or, at most, a pint. Both 

 necks are fitted with new 

 perforated corks, which must 

 be perfectly tight Through 

 one of these the funnel-tube 

 b is passed air-tight, and 

 through the other the bent tube c, which is expanded at e into a bulb about an 

 inch in diameter. This bulb serves to collect the particles of liquid which are thrown 

 up from the contents of the bottle, and which drop again into the latter from the 

 end of the tube. The other end of the tube is connected, by means of a cork, with the 

 tube d, about six inches long, which is filled with fused chloride of calcium, free from 

 powder, destined to retain the moisture. In the opposite end of the tube d is fixed, 

 air-tight, another tube, e, made of glass free from lead, 12 inches long, and, at most, 

 pth of an inch in internal diameter. It must be observed that the funnel-tube d is 

 indispensably necessary to introduce the fluid to the pieces of perfectly pure metallic 

 zinc already placed in the bottle. Hydrogen gas is at once formed, and if arsenic be 

 present, in oven the smallest quantity, it combines with the hydrogen, and escapes 

 as arseniuretted hydrogen. If the gas as it issues from the jet is set on fire, no 

 product but water is generated if the hydrogen be pure ; and by holding against the 

 flame a cold white porcelain basin, or piece of glass, or of mica, no steam is produced, 

 and a dew is formed upon the cold surface. If arsenic be present, a deposit is ob- 

 tained, which, according to the part of the flame in which the substance to receive it 

 is placed, will be either a brown stain of metallic arsenic, or a white one of arsenious 

 acid. If the quantity of arsenic is too small to be detected in this way, it will be well 

 to ignite the horizontal part of the tube. All the arseniurotted hydrogen will, in 

 passing that point, become decomposed, and deposit its arsenic. The heat will drive 

 this forward, and a little beyond the heated portion metallic arsenic will be con- 

 densed. Several precautions are necessary to be observed ; but for the details of 

 those we must refer to works especially directed to the consideration of this subject 

 One source of error must, however, be alluded to. A compound of antimony and 

 hydrogen is formed under similar circumstances ; and this gas in many respects re- 

 sembles the compound of arsenic and hydrogen. If the stain formed by the flame 

 is arsenic, it will dissolve, when heated, in a drop or two of sulpho-hydrido of am- 

 monia, and a lemon-yellow spot is left ; if antimony is present, it leaves a yellow 

 stain. Wohler. 



If a drop of bromine is placed on a saucer, and a capsule containing arsenical spots 

 inverted over it, the spots take a very bright lemon-yellow tinge in a short time. Anti- 

 monial spots, under the same circumstances, are acted on much more rapidly (in about 

 five seconds at a temperature of 52 F.), and assume an orange shade. Both become 

 colourless if exposed to the air, and are again restored if treated with a strong solu- 

 tion of sulphuretted hydrogen. The secondary yellow of the arsenical spots, as 

 observed by Lassaigne, disappears on the addition of ammonia, whilst that of the an- 

 timonial spots remains untouched. A concentrated solution of iodato of potash turns 



