46 Dr John Davy on Oxide of Arsenic. 
specimens tried, not varying more than one-tenth of a grain. 
In each instance this residue was found to consist chiefly of 
common salt; it tasted of this salt, and in solution was 
copiously precipitated by nitrate of silver. Mixed with the 
common salt was a little magnesia and lime, both probably 
in combination with sulphuric acid, as sulphate of lime and 
of magnesia, the presence of the acid being denoted by 
nitrate of barytes; a trace, too, of oxide of arsenic was 
detected in each, and it may be inferred, in combination 
with potash, a trace of which also was obtained. The 
arsenic was detected not only by the test of the ammoni- 
aco-nitrate of silver, but also by reduction to its metallic 
state by sublimation, after mixture with ferro-cyanide of 
potassium. Of the several specimens of water tried, that 
procured in October, when the stream was about its ordinary 
size, afforded a. somewhat stronger trace of the metal than 
either the earlier or later, the one in August 1861, taken 
when the stream was swollen after rain; the other in Jan- 
uary 1862, during a frost of several days’ duration, when 
probably the water was frozen at its sources,—the stream 
then lower than common. In noticing, however, the later, 
I should except the last, that taken in August last, when 
the stream was of about its average volume, and the indica- 
tions of arsenic nearly the same as those of October. In the 
instance that the trace was strongest, judging from com- 
parative experiments with oxide of arsenic,—experiments 
of reduction by sublimation,—the quantity of oxide con- 
tained in the pint was only about ‘008 grain, or ‘064 
grain to the gallon. 
Before concluding this part of my subject I may mention 
that in the second visit which I made to the stream, then 
in company with Dr Robinson of Newcastle, we paid par- 
ticular attention to the herbage and plants growing close to 
or in the water, and were unable to observe anything in their 
appearance indicative of a noxious influence on them, with 
the exception—if it be an exception—of a single tree, a 
maple (Acer campestre), growing where its roots were 
washed by the descending stream; of its branches many 
were dead, and such was its general aspect that it might 
well represent a poisoned tree; yet, that it was poisoned, is 
