to Dr. Wollaston. 
109 
mercury having a great affinity for prussic acid, the presence 
of that metal in the system might prevent the effect in ques- 
tion. But as in the two other failures, no mercury was pre- 
sent, I cannot lay any stress upon that conjecture. It may be 
proper to mention, that in the frequent trials which I have 
made with the prussiat of potash, no symptom or inconveni- 
ence whatever has ever occurred which could be ascribed to 
that salt. 
I remain ever, 
my dear Sir, 
with great esteem, your’s sincerely, 
ALEX. MARCET, 
P. S. Whilst revising the proof of this sheet, it has been 
observed to me by some friends, and in particular by Dr. 
Henry of Manchester, and Dr. R. Pearson of London, that 
in order to show distinctly that certain substances find their 
way to the bladder, without passing through the general cir- 
culation, it would be necessary to examine the arterial, as well 
as the venous blood, since it is not impossible that the whole 
of the sugar in diabetes, or the prussiat of potash in the expe» 
riments above related, may be conveyed to the urinary organs 
by the arteries, without entering the venous system. Ac- 
cording to this hypothesis, it may be conceived that the same 
substances when conveyed by the arteries to distant parts of 
the body, may return by the absorbent system, and might in 
that case be discovered in the thoracic duct. This view of the 
subject may deserve further investigation ; and I hope that this 
curious question will soon be decided by appropriate experi- 
ments. 
