Davy — On Arsenical Compounds. 227 



to sodium, I am not aware that arsenic has ever been pointed out as one 

 of its impurities ; and, as to its presence in mercury, that is, I believe, 

 a cii'cumstance of very rare occurrence ; but, should it exist in that metal 

 as an impurity, it can be readily removed from it by digesting the 

 mercury in diluted nitric acid, and afterwards well washing it with 

 water. The amalgam which I have found to answer very well for the 

 detection of arsenic, consists of one part by weight of sodium to eight or 

 ten parts of mercury, and is easily made by heating moderately in a test 

 tube over a lamp the mercury, and then adding gradually in small 

 pieces the sodium, taking care to keep away the face, if unprotected 

 fi'om the mouth of the tube, lest some of that metal in an ignited state 

 might be spirted out during the addition of the first portions. Those 

 metals readily combine under these circumstances, forming an alloy 

 that is Kquid whilst hot, but becomes hard and brittle when cold. 

 The contents of the tube, while still hot and liquid, are quickly 

 poured out on a clean plate, and, when cool, broken up in small 

 lumps, which are then immediately placed in a well corked or stoppered 

 bottle. 



The way I employ this amalgam is simply to place the suspected 

 solution, or solid matter along with a little water, in the bottom of a 

 test glass, then add a small bit of the amalgam, about the size of a 

 grain of wheat, and lastly, place without delay, on the top of the 

 glass, a piece of white filtering paper or the cover of a white porce- 

 lain crucible moistened with a drop of a dilute solution of nitrate of 

 silver, slightly acidulated with nitric acid, when if. arsenic is present, a 

 dull black or deep brown stain on the paper, or a dark silvery one on the 

 porcelain, will be quickly developed in the part moistened, owing to 

 the silver of the salt being reduced to the metallic condition by the 

 agency of the arseniuretted hydrogen thus evolved, which, coming in 

 contact with the nitrate of silver, gives rise to the following reaction : 

 H3AS + 6 AgNOa + 3H,0 = 6 HXO3 + TI3ASO3 + 3 Ag.,. 



The silver solution, which I have found to answer very well for 

 this pui-pose, was made by dissolving twenty grains of the nitrate in 

 an ounce of distilled water, and then adding two drops of strong nitric 

 acid, to render the solution slightly acid. I may further add that I 

 generally place a small disc of bibulous paper between the mixture in 

 the glass and the paper or cover moistened with the silver solution, to 

 intercept any particles of the liquid which might otherwise be projected 

 against them, producing there minute black spots, and thus interfering 

 with the results of this test. 



I have found that exceedingly minute quantities of arsenic can be 

 readily detected by this very simple process ; thiis the one thousandth 

 part of a gi'^in of arsenious acid, dissolved in one cubic centimetre of 

 distilled water, gives a very decided effect in a few moments ; but much 

 smaller c[uantities are detectable by it ; thus the one hundred thou- 

 sandth or even the one millionth part of a grain of arsenious acid, dis- 

 solved in the same quantity of water (one cubic centimetre), will 

 afford, bv the blackening of the silver salt, after a little time, an indi- 



