8 Dr. Thomas Thomson * [July 



stracted by the kidneys and thrown out of the system. 

 This is brought about by making them soluble in water, 

 and for this purpose they are converted into phosphoric 

 acid, sulphuric acid, lime and magnesia, substances which 

 in certain states of combination are soluble in water and 

 consequently in urine. Uric acid is also formed and 

 rendered soluble by being united with ammonia. It would 

 seem that ammonia is also formed in the kidneys, at least 

 we have no evidence that it exists in the blood. The obser- 

 vations of Dr. Marcet confirm the previous statement of 

 Proust, that carbonic acid is also a frequent constituent of 

 urine. 



The most remarkable substance in urine is the body to 

 which Fourcroy and Vauquelin gave the name o^ urea. It 

 was originally detected by Rouelle junior ; but it is chiefly 

 by the researches of Cruickshanks and Dr. Prout, that its 

 characters have been determined with accuracy. Accord- 

 ing to Berzelius it exists in healthy urine to the amount of 

 three per cent. But I have never been able to extract so 

 much, except in certain cases where the quantity of urea 

 was obviously in excess. My method was to mix the urine 

 with the requisite quantity of nitric acid, and concentrate 

 by spontaneous evaporation in a very dry and warm place 

 till as much nitrate of urea fell in crystals as could be got. 

 Weighing this precipitate and determining its solubility in 

 water, I calculated the portion remaining in solution in 

 the residual liquid ; and knowing the urea in a given weight 

 of this nitrate, I concluded the whole urea in the quantity 

 of urine operated on. By this method I extracted from 

 1000 grains of healthy urine of the specific gravity 1*0185, 

 23.64 grains of urea. I employed several other methods 

 but none of them gave such good results as the one just 

 described. 



Except the acid or rather the supporter which exists 

 in common salt, no acid can be detected in blood, but a 

 trace of phosphoric. In urine, however, there are always 

 three acids, namely, the uric, phosphoric, and sulphuric, 

 and three bases, namely, lime, magnesia, and ammonia, 

 which cannot be demonstrated in blood. The great function 

 of the kidney then is the formation, or at least the separa- 

 tion of certain acids and bases. The acids and bases are 



