110 Prof. Davy on the presence of Arsenic 



Having collected the stalks, leaves, and pods of the peas, I care- 

 fully kept them for examination, to ascertain if those plaiits Under 

 the treatment they had been subjected to had taken up any arsenic. 

 Professional business, however, of one kind or another prevented 

 at the time my pursuing the subject any further ; and I did not 

 resume the inquiry till recently, when, being engaged in the de- 

 tection of arsenic in a case of suspected poisoning, my attention 

 was again called to this subject. 



In the case I allude to, the quantity of arsenic present in the 

 stomach and its contents was very minute, and I had recourse to 

 several methods for the detection of that metal before I could 

 affirm positively as to the existence of arsenic ; and I found that 

 by employing conjointly Reinsch's and Marsh's methods, by far 

 the most satisfactory results were obtained. 



These methods are well known. The first consists in boiling 

 the suspected substance along with diluted hydrochloric acid, 

 together with some pieces of metallic copper, Avhen, if arsenic is 

 present, it will be deposited in the metallic state on the surface 

 of the copper, giving it a peculiar steel-grey appearance ; and on 

 heating the copper after being washed and dried, the arsenic can 

 be volatilized as arsenious acid, and identified by its appropriate 

 tests. The second method consists in bringing the suspected 

 substance, in a state of solution, in contact with a mixture of 

 zinc and diluted sulphuric acid contained in a suitable apparatus) 

 when the arsenic, if present, will combine with the hydrogeii 

 being generated, and will form arseniuretted hydrogen, a gaseous 

 compound which is characterized by its producing a stain of 

 metallic arsenic when any cool surface is held over a srtiall jet Of 

 the gas whilst burning. On trying by these methods the stalks 

 and leaves of the pea-plants which I had watered with arsenious 

 acid, I found that arsenic could be readily detected in them, and 

 was present even in the seeds ; showing clearly that arsenic had 

 been freely taken up by those plants, and that every portion of 

 them appeared to have imbibed the poison. 



This experiment having shown me that arsenic might be taken 

 up in considerable quantity by plants without its destroying their 

 vitality, or appearing even to interfere with their proper func- 

 tions, I proceeded to ascertain if the arsenic, as it existed in dif- 

 ferent artificial manures (such as the superphosphate), would in 

 like manner be taken iip by plants growing where those manures 

 had been applied. To determine this, I transplanted, last April, 

 a small cabbage-plant into a flower-pot in which I had previously 

 put a mixture of one part of superphosphate to four parts of 

 garden mould. The cabbage after a short time appeared to 

 recover the transplanting ; and when it had been growing in the 

 mixture for three weeks, I cut off the top of the plant, which 



