1890.] MICROSCOPICAL JOURNAL. 157 



Detection of Arsenic by the Microscope. 



By E. a. GIBBS, 



WASHINGTON, D. C. 



Arsenic (arsenious oxide) or " white arsenic" presents a characteristic 

 octahedral or derivative of octahedral crystal, varying from ^4-^ to -f,-^^-^ 

 of an inch in diameter. The powder found in the shops is composed 

 of either an amorphous mass or of crystals, hence the microscopical 

 character of the powder, whether crystalline or not, the relative pro- 

 portion of crystalline to amorphous matter and the prevailmg size of 

 crystals or lumps may, in some instances, enable us to determine with 

 considerable certainty whether or not a sample was obtained from a 

 given source. It has been found by certain observers that great uniform- 

 ity generally exists among samples of powder taken from the same 

 source. 



The crystals of arsenious oxide are readily obtained by volatilizing 

 metallic As in a free supply of air. In cases of arsenical poisoning, 

 owing to the difficult solubility of arsenic, white particles may be found 

 in vomited matter or adhering to the walls of the stomach and intes- 

 tines. These should be carefully picked out, washed, and dried, and 

 then introduced into a reduction tube. A small piece of charcoal is 

 placed above ; then heat applied first to the charcoal until red hot. 

 Then hold the whole end of the tube in the flame. The arsenic will 

 be reduced to the metallic state and deposited a little further up the 

 tube. Shake out the remains of the charcoal and heat the metallic 

 ring when it will volatilize, and, combining with the oxygen of the 

 air, be deposited in the form of a white ring composed of minute octa- 

 hedral crystals. 



For liquids, freed from organic matter, the " Marsh process '" appears 

 to be the most certain. The action of this test depends on the decom- 

 position of the soluble compounds of arsenic by nascent hydrogen and 

 the formation of arseniureted hydrogen, which when decomposed by 

 heat yields a deposit of metallic arsenic on the cool portion of the tube. 

 When a portion of this sublimate is heated in a tube the deposit of 

 crystals occurs. It is said that y^pj-^ ^ gr. of Asg O3 in i. 000000 of 

 water yields quite a good sublimate. 



Fallacies of this test are, first, presence of As. in reagents Zn and Hg 

 SO^, which must be determined beforehand. Presence of antimony in 

 the suspected substance, which also yields a metallic stain by the Marsh 

 process. But the stain will be found on both sides of the heated tube, 

 owing to the fact that antimonureted hydrogen is decomposed at a 

 lower temperature. Arsenic is only found in advance of heat. In a 

 tube, arsenic when heated, decomposes readily, and recondenses a little 

 further on in crystalline form. Antimon}- requires a much higher tem- 

 perature, but also deposits sometimes in octahedral crystals. The 

 identity of these crystals can readily be established by chemical tests. 

 Where both are present advantage may be taken of the use of a bath of 

 olive oil, which boils at about 600° F. ; arsenic volatilizing completely 

 between 400° and 500° F. and antimony requiring a much higher tem- 

 perature. 



Reinsh's process is another method. The suspected substance being 

 dissolved in distilled water, to which about \ of its volume of H CI has 



