566 
Proceedings of the lioyal Society 
will admit ; so many nitrogenous substances exist in it : these are 
closely allied one to another in their chemical relations, hence their 
separation is very difficult; and, lastly, during the process of separ- 
ation, one substance may be changed into another. 
In the case of urea estimation, fresh obstacles stand in our path 
which it is necessary to understand, in order that their removal may 
be attempted. Urea not only entirely decomposes when heated 
over 120° C., but when a watery solution is evaporated to dryness, 
part of it decomposes, producing a loss which varies of course 
with the quantity of water and the strength of the solution. A 
fraction of a gramme evaporated in a litre of water loses from 3 
to 4 per cent. How it is necessary to separate the albumen of 
blood from the urea, which entails the addition of much fluid, 
which fluid has to be evaporated down when decomposition of part 
of the urea ensues. 
A common way is to coagulate the albumen with hot alcohol 
when three volumes of spirit are at least required. With acidulated 
boiling wuter, six or seven volumes are necessary for the complete 
coagulation. Besides this, the decomposition on evaporation is 
much increased if other organic impurities are present in the fluid. 
So much so, that if ordinary defibrinated undiluted blood be eva- 
porated in flat dishes, even with a gentle heat, not a trace of urea is 
to be discovered in the hard black cake which results. Hay, a 
large quantity of pure urea may have been previously added, the 
whole decomposing during the evaporation. This is also the case 
with the watery and alcoholic extracts of urea from blood, for these 
contain much extractive matter of which the urea forms but a small 
portion. The loss which occurs during these evaporations is far 
more than would occur were the urea alone present in an alcoholic 
or watery solution. 
Another difficulty in our way is that no substance was known 
which might be useful in its extraction, and in which it is insoluble. 
It is often thought to be insoluble in sulphuric ether, but this is 
far from the truth ; indeed it is so soluble that ether can never be 
used to separate, say fat from it, in an analysis which professes 
to be quantitative. Urea, it may be stated, is very soluble in 
water and alcohol, and is soluble also in chloroform and acetic 
ether. 
