EXPERIMENTS WITH URIC ACID. 219 



6. The chief interest which attaches to experiments with 

 uric acid arises from the circumstance, that while, in man, the 

 nitrogen separated from the system by the urine escapes chiefly 

 in the state of urea, which readily decomposes, birds, serpents, 

 and insects void it in the state of uric acid, which is a much 

 more stable compound. This is one reason why the urine of 

 man so readily produces ammonia, and why the excretions of 

 birds are more capable of lengthened preservation.* Our na- 

 tural guanos owe to this cause their richness in ammoniacal 

 matter, and it will be very useful to know, whether or not 

 plants can directly avail themselves of uric acid without pre- 

 vious fermentation or decomposition. We cannot ask such 

 experiments at the hands of the practical farmer, but there are 

 many at whose command uric acid, artificially prepared or 

 excreted by serpents, may be in sufficient quantity to admit of 

 its being employed for experiments upon vegetation. 



7. The experiments of Kuhlmann and Lawes, made with rape- 

 dust and with gelatine, of which an account has been given in 

 the early part of this chapter, suggest various inquiries, which 

 the experimenter will desire to answer by his trials with these 

 substances, such as 



a Do gelatine, sulphate of ammonia, and nitrate of lime, 

 applied in quantities containing equal weights of nitrogen, 

 always approximate in their effects, as in Kuhlmann's experi- 

 ment upon hay, and are they always surpassed by nitrate of soda? 



b Do rape and other cakes generally produce a greater 

 comparative effect upon turnips than the sulphate of ammonia 

 does, in proportion to the nitrogen they contain, as in Mr Lawes' 

 experiment, or a smaller effect, as in Mr Fleming's (p. 193) ? 



* The relative composition of urea and uric acid is as follows : 



Urea. Uric acid. 



Carbon, 20.0 37.15 



Hydrogen, 6.6 2.49 



Nitrogen, 46.7 34.66 



Oxygen, 26.7 25.70 



100. 100. 



In twenty-four hours, a full-grown man voids about 270 grains of urea, and 

 only about 8 grains of uric acid. 



