UREA. 159 



ciently heated at a single spot, the whole mass assumes a phos- 

 phorescent appearance ;) from this glowing residue the cyanate of 

 potash which has been formed must be extracted with cold water, 

 and mixed with 20J parts of dry sulphate of ammonia ; most of 

 the sulphate of potash separates in a crystalline form, while the 

 cyanate of ammonia, now converted into urea, remains in solution. 

 The remaining sulphate is separated by crystallisation, but more 

 perfectly by alcohol. 



Tests. Urea may generally be very easily recognised by its 

 properties, especially by its behaviour towards nitric and oxalic 

 acids ; but when we have to discover very minute quantities of this 

 substance in albuminous fluids, it is often very difficult to determine 

 its presence with scientific precision. It is in alcoholic extracts 

 that we must always seek for urea, but before we proceed to search 

 for it, there are several precautionary measures to be adopted, the 

 neglect of which would render our attempt to discover it futile. 

 In the first place, in reference to the presence of albuminous 

 substances, if we wish to discover small quantities of urea in 

 albuminous fluids, we must not be satisfied with the removal of the 

 albumen by simple boiling ; since by the coagulation of the albumen 

 the fluid becomes more alkaline, and might, during evaporation, 

 induce a decomposition of the urea ; moreover, all albuminous 

 matter is not precipitated by boiling, but a portion remains dissolved 

 by the alkali, and is taken up in the alcoholic extract ; on evapo- 

 ration this albumen undergoes a change which probably cooperates 

 with the alkali in inducing the decomposition of the urea. This may 

 explain how it was that Marchand could only recover 0'2 of a gramme 

 of urea from a mixture of 200 grammes of serum and 1 gramme 

 of urea. Hence, before boiling the albuminous fluid, we must add 

 a few drops of acetic acid, so as to give it a slightly acid reaction, 

 whereby not only is the alkalescence of the fluid prevented, but a 

 much more perfect separation of the coagulable matters is effected. 

 If the residue of the fluid from which the coagulated matters have 

 been filtered be extracted with cold alcohol, and the solution rapidly 

 evaporated, so as to cause the chloride of sodium (taken up by 

 the cold alcohol) to separate as much as possible in crystals, on then 

 bringing a drop of the mother-liquid in contact with nitric acid 

 under the microscope, we shall observe the commencement of the 

 formation of the rhombic octohedra, and the hexagonal tablets, in 

 which, if the investigation is to be unquestionable, the acute angles 

 (=82) must be always measured. After the determination of the 

 nitrate we may also obtain the oxalate, and submit it to microscopic 



