103 
of Sugar in the Blood. 
to ascertain, by giving large doses of nitre, which he could 
perceive to pass with the urine, but could not detect in its pas- 
sage through the blood ; and he imagined the channel by 
which it was conveyed to be the absorbent system, upon the 
supposition that they might admit of a retrograde motion of 
their contents. 
Without adopting the theory of Dr. Darwin, it did appear 
to me that the fact deserved to be ascertained by some test 
more decisive than nitre, and I conceived that if prussiate of 
potash could be taken with safety, its presence would be dis- 
cerned by means of a solution of iron in as small proportion 
as almost any knov/n chemical test. Upon trial of this salt, I 
found that a solution of it might be taken without the least 
inconvenience, and that in less than one hour and a half the 
urine became perceptibly impregnated, and continued so to 
the fifth or sixth hour, although the quantity taken had not 
amounted to more than three grains of the salt. 
After a few previous trials of the period when the principal 
impregnation of the urine might be expected, and when the 
presence of the prussiate (if it existed in the blood) might 
with most reason be presumed to occuf s a healthy person 
about thirty-four years of age was induced to take a dose cor- 
responding to three grains and a half of the dry salt, and to 
repeat it every hour to the third time. The urine being exa- 
mined every half hour, was found in two hours to be tinged, 
and to afford a deep blue at the end of four hours. Blood 
was then taken from the arm, and the coagulum, after it had 
formed, was allowed to contract, so that the serum might be 
fully separated. The presence of the prussiate was then en- 
deavoured to be discovered by means of a solution of iron, but 
