78 Dr. Rainy on the Detection of Arsenic. 



It is generally supposed that this process is equally applicable to all the 

 couipouncls of arsenic soluble in dilute muriatic acid ; and that in all cir- 

 cumstances it detects the presence of the metal, with a delicacy more 

 than sufficient for every practical purpose. 



Soon after the publication of Reinsch's method, I made various experi- 

 ments, with the view of determining the limits within which its indications 

 might be relied on. The result was unsatisfactory ; for while, in some 

 cases, it appeared to be fully as delicate as the method of IMarsh, in other 

 cases I failed to obtain the metallic deposit whei'e the arsenic was present 

 in a much higher proportion. Similar observations have been made by 

 others ; for it is stated by Fresenius and Von Babo that, " the presence 

 of nitrates and various salts of mercury, and other metals, render the 

 separation of arsenic by copper difficult or even impossible." It seems 

 also to be a general opinion, that when the proportion of ai'senic is ex- 

 tremely minute, the process of Marsh is decidedly preferable. 



It is obviously important that the cause of such discrepancies should 

 be investigated, as the great simplicity and rapidity of Reinsch's process 

 render it peculiarly suitable for medico-legal investigations, and give it a 

 decided superiority over every other, if it can be conducted in a manner 

 that wUl ensure equal delicacy. 



The following experiments were made with the view of ascertaining the 

 cause of these discrepancies, and, if possible, the means of preventing 

 them. The copper was used in the form of very thin foil, which was 

 easily cleaned and polished, so as readily to show any change of colour ; 

 the fluid usually contained one tenth part, by measure, of muriatic acid 

 of the ordinary strength, except when the object was to ascertain the 

 effects of varying this proportion ; and in order to prevent any diminution 

 of the fluid, or any change in its strength during the boiling, a condenser, 

 containing cold water, was placed closely over the mouth of the vessel i 

 which the process was carried on. 



1. My first object was to ascertain the exteiit of copper surface that 

 can, in the most favourable circumstances, be distinctly coated by a given 

 quantity of arsenic. The results were very uniform. One thousandth of 

 a grain of arsenious acid gave a full steel colour to one square inch of 

 copper surface. When tioo square inches of copper are used with the 

 same quantity of arsenious acid, the eficct is stiU distinct ; but the de- 

 posit is, in these circumstances, so thin, that there is a tinge of yellow, 

 apparently from the copper shining through, or not being uniformly coated. 

 Two square inches is the utmost extent of copper surface that can be dis- 

 tinctly coated by one tJiousandth of a grain of arsenious acid ; and it can 

 be proved that, in these circumstances, the thickness of the film of deposited 

 metal does not exceed ^..oon.oijo (one-four millionth) of an inch. 



It follows from this result, that if the extent of copper surface be too 

 great relatively to the arsenic present, no distinct deposit will be obtained ; 

 thus, a fluid containing one thousandth of a grain of arsenic, with th7'ee 

 square inches of copper surface, might give a tarnish, but no distinct coating. 



