104 Dr. Wollaston on the Non-existence 
without effect ; and as I thought that the redundant alkali 
(which had been ascertained to prevail in this serum) might 
tend to prevent the appearance of the precipitate, I added a 
small quantity of dilute acid ; but still I could not discern that 
any degree of blueness was occasioned by it. 
This experiment having been repeated a second time with 
the same result, seemed to me nearly conclusive with respect 
to the existence of some passage, by which substances cer- 
tainly known to be in the stomach may find their way to the 
bladder without being mixed with the general mass of circu- 
lating fluids. 
Being desirous of ascertaining whether the prussiate could 
be discovered in any other secretions, I have repeatedly exa- 
mined my saliva, at times when the urine has manifested a 
very strong blue, by adding solution of iron, but I could at no 
time perceive the saliva to be tinged. 
I have also, during a severe cold, accompanied with profuse 
running of water from the nose, made a similar examination 
of this discharge, but have not been able to perceive any trace 
of the prussic acid. 
It was nearly in this state that I left the inquiry at the pe- 
riod I have mentioned, and I do not remember to have made 
any other experiments, when I requested your assistance in 
making trial of the serum that is secreted in consequence of 
the application of a blister. Your report upon the result of 
your experiments, in addition to those which I have above 
related, nearly satisfied me as to the existence of some un- 
known channel of conveyance by which substances may reach 
the bladder. 
With respect to Dr. Darwin’s conception of a retrograde 
