565 
of Edinburgh, Session 1879-80. 
watery solution of fair strength is not difficult; nothing, indeed, can 
well he easier. There are a number of methods which we might 
employ, and which are exact and easy of application ; such are 
those of Liebig, of Heintz, and Ragsky, of Bunsen, of Huefner, 
and of Davy, all of which may he applied for its estimation 
in urine. 
With the blood it is another question, for here this substance 
exists in very small quantities (3 parts in 10,000), mixed with a 
mass of organic matter, from which it has first to be separated in a 
tolerably pure form before the quantity can he ascertained. This 
separation from the blood is the difficult task to be overcome, the 
estimation of the quantity will then he easy. 
The subject is one of great importance in physiological 
chemistry, as all will admit, for it is a key to our knowledge of 
the metabolism of albuminous substances in the body, nearly all 
the nitrogen which is excreted passing out in this form. 
This importance has been thoroughly appreciated by chemists, 
and many men of note have turned their attention to this sub- 
ject. 
It was Sir Robert Christison who first gave the stimulatus to 
modern inquiry, for he it was who found a large amount (much 
above the normal) of urea in the blood of patients suffering from 
Bright’s disease. 
His method did not pretend to great chemical accuracy, for he 
took only the clear serum of the blood, from which he crystallised 
out the urea as a nitrate after concentration. 
This discovery ranks with the highest that chemists have made 
in their investigations of the healthy and diseased tissues, and few 
indeed have been the facts gleaned gradually, and with difficulty in 
later years. 
Physiologists have endeavoured, hut in vain, to found an 
accurate analytical process for its determination in blood, in 
order to investigate the many changes which occur in its amount 
during different physiological and pathological conditions. 
Lecanu, Marchand, Simon, Millon, Pettenkofer, Joseph Picard, 
Gscheidlen, and Drechsel have all worked with this object in 
view. 
Indeed the chemistry of blood is beset with difficulties, as all 
