177 [Lar. 
Urea is peculiar to the urine: is produced by the 
action of the kidnies on the blood. Dr. Prout 
suggests that they get it from the albuminous 
matter. 
Lithic or uric acid, is not found in the blood, but 
is always present in healthy urine. Majendie 
thinks it is obtained from the azote of ali- 
mentary substances. Dr. Prout thinks it is 
always in combination with ammonia, (lithate 
of ammonia) from which acids, even carbonic, 
separate it in the fourm of a red powder. - , 
Erythic acid is a peculiar acid so named by M. 
Brugnatelli, obtained by treating pure lithic 
acid with diluted nitric acid, applying a moder- 
- heat. Itis in transparent colourless crys- 
tals. 
Purpuric acid ; this is obtained from the the Ery- 
thic acid crystals, treated by Prout, while ina 
state of boiling hot solution, 
stals 
ric acid, yield pure purpuric acid in a yellowish 
powder. . ore 
The phosphoric acid and its compounds are active 
agents in the production of calculi. This acid 
often exists in a free state in the urine. In 
_ healthy urine it is in union with soda and ammo- 
monia, and partly with lime and magnesia ; 
these last being retained in solution by excess of 
Dr. Prout, divides the mechanical d 
the urint, in three clauses: = 
1. Pulverulent, or Amorphous sediments. 
2. Chrystalline sediments (called gravel.) 
3. Solid concretions, (or calculi formed by con- 
