ioB 
Dr. Marcet’s Reply 
Experiment 3. 
“ Dec. 2, 1807. The fluid obtained by means of a blister 
(as in Experiment 1,) being not immediately derived from 
the circulation, since it may be considered as the product of a 
secretion, I was desirous of repeating Dr. Wollaston's expe- 
riment on the serum itself, under circumstances of impreg- 
nation similar to those in which the serum of the blister was 
examined. 
“ For this purpose, a young woman after taking, in divided 
doses, about a dram of prussiat of potash in the course of twelve 
hours, lost some blood by cupping, an operation which had 
been ordered for a local complaint under which she laboured. 
The serum having been allowed to separate, and a little nitric 
acid having been added to it, not the least vestige of prussic 
acid appeared in applying the test of sulphat of iron, although 
the urine made during the six hours which preceded and fol- 
lowed the cupping, was strongly impregnated with that add, 
and struck a vivid blue upon adding the smallest quantity of 
iron/' 
I have only to observe, in addition to these particulars,that the 
susceptibility by which prussiat of potash is transmitted to the 
bladder, seems to vary in different individuals; for in five trials, 
made at Guy’s Hospital in Nov. 1805, I failed of discovering 
any vestige of that salt in the urine of persons who had taken 
it in quantities sufficient to produce its appearance in others. 
Three of these individuals, I should observe, were at the time 
under mercurial treatment, and an idea occurred to me that 
