426 Intelligence and Miscellaneous Articles. 



ment of precipitation, by the nitrous acid produced during the solu- 

 tion of the silver in the nitric acid ; manganate of potash was therefore 

 added to a solution of a gramme of silver and six milligrammes of 

 mercury, as long as it was decolorized, and even slightly in excess ; 

 but the result was not satisfactory: the quantity of the silver was in- 

 creased by about five thousandths. 



It therefore remained to discover the means of ascertaining the 

 presence of mercury in silver j and this was effected by observing the 

 manner in which chloride of silver when pure, and when adulterated 

 with mercury, is affected by light. 



It is well known that chloride of silver blackens the more readily as 

 it is exposed to an intense light, and that even in the diffused light of 

 a room it becomes soon sensibly coloured. If it contains four to five 

 thousandths of mercury it does not blacken, it remains of a dead white; 

 with three thousandths of mercury there is no marked discoloring 

 in diffused light ; with two thousandths it is slight j with one, it is 

 much more marked, but still it is much less intense than with pure 

 chloride. With half a thousandth of mercury the difference of colour 

 is not remarkable, and is perceived only in a very moderate light. 



But when the quantity of mercury is so small that it cannot be de- 

 tected by the difference of colour in the chloride of silver, it may be 

 rendered very evident by a very simple proeess of concentration. 

 Dissolve one gramme of silver, supposed to contain J of a thousandth 

 of mercury, and only one fourth of it is to be precipitated, adding 

 only % of the common salt necessary to precipitate it entirely. In 

 thus operating, the J thousandth of mercury is concentrated in a 

 quantity of chloride of silver four times smaller: it is as if the silver 

 having been entirely precipitated, four times as much mercury, equal 

 to two thousandths, had been precipitated with it. 



In taking two grammes of silver and precipitating only ^ by com- 

 mon salt, the precipitate would be, with respect to the chloride of 

 silver, as if it amounted to four thousandths. In this process, which 

 occupies only five minutes, because exact weighing is not necessary, 

 -rV of a thousandth of mercury may be detected in silver. 



It is not useless to observe, that in making these experiments the 

 most exact manner of introducing small quantities of mercury into a 

 solution of silver is to weigh a small globule of mercury, and to dis- 

 solve it in nitric acid, to dilute the solution so that it may contain as 

 many cubic centimetres as the globule weighs of centigrammes. 

 Each cubic centimetres, taken by means of a pipette, will contain one 

 milligramme of mercury. 



If the ingot of silver to be assayed is found to contain a greater 

 quantity of mercury, one thousandth for example, the humid process 

 ought either to be given up in this case or to be compared with cupel- 

 lation. 



When the silver contains mercury, the solution from which the 

 mixed chlorides are precipitated does not readily become clear. 



Silver containing mercury, put into a small crucible and mixed 

 with lamp-black, to prevent the volatilization of the silver, was 

 heated for three quarters of an hour in a muffle, but the silver in- 

 creased sensibly in weight. This process for separating the mercury 



