241. ESTIMATION. 247 



ing in a pneumatic trough, should be capable of holding 50 to 60 

 cc., and be graduated to 0'2 cc. It is filled with the iron-solution 

 contained in B by opening the clip h and blowing through the 

 shorter bent tube in B ; by this means the solution can be run into 

 the pneumatic trough ; on opening / and sucking at g the solution 

 rises in the tube until it reaches and passes /, which should then 

 be closed. 



After replacing the clip h, the bent point d is introduced under 

 the measuring tube and the solution of the amide allowed to run 

 from the second funnel-tube into the generating vessel, rinsing with 

 a little water, but keeping the tube from e downwards full of 

 liquid. Small quantities of sulphuric acid are allowed to run into 

 the generating vessel from time to time, when the evolution of gas 

 becomes sluggish, taking care that the measuring-tube always 

 contains sufficient strong solution of ferrous sulphate ; this can be 

 ensured by frequently opening the clip h and allowing the solution 

 from B to run into the measuring-tube. The end of the decom- 

 position is recognised by the liquid in A assuming a permanent 

 blue colour from excess of nitrous acid. The remainder of the gas 

 is then driven out by filling the entire apparatus with water 

 through the second funnel-tube until it flows into the measuring- 

 tibe through d. The delivery-tube is now removed, and the 

 vhole of the nitric oxide absorbed by the introduction of fresh 

 iron-solution. After closing the clip h, the delivery-tube from B 

 is drawn out of the measuring-tube, and the latter transferred to a 

 deep cylinder, where the iron-solution is removed as far as possible 

 and replaced by caustic soda to absorb carbonic acid. When this 

 has been effected, the measuring-tube is lowered in the cylinder 

 until both liquids have the same level The volume of gas is now 

 read off, reduced to and from it the amount of amide originally 

 present calculated, deducting 1 cc. as unavoidable error caused by 

 the atmospheric air mixed with the nitrogen ; 28 parts by weight 

 of nitrogen indicate 150 of crystallized asparagine, 131 of leucine, 

 and 181 of tyrosine. ( 191, 192). 



242. Amidic Acids. The amidic acids referred to in 101 are 

 freely soluble in water and 50 per cent, spirit, requiring consider- 

 able quantities of strong alcohol for precipitation, so that in this 

 respect they resemble such substances as dextrin, levulin, etc. 

 They are precipitated therefore with, or in the place of, dextrin 

 and the like, but differ from these bodies in containing nitrogen. 



