Then, if the test-tube and its contents be slowly cooled, at the end of about twelve hours the sediment is found to consist partly of spherical resinous lumps, and partly of microscopical tuft-shaped crystals.
These crystals are nothing else than glycosazone-compounds. (Moritz.) In normal urine this reaction is very often observed as distinctly evident as if we had to do with a urine containing as much as 1 to 2 per cent. of sugar.
In reality besides the extremely minute traces of grape-sugar, a whole number of aldehydes and ketones are present in urine, which can form phenylazone.
Amidst all these substances those which, according to the investigations of Flückiger chiefly interfere with these tests are the glycuronic acid compounds, as they give crystals of the same form in the course of the reaction. Flückiger detected these compounds by their property of rotating the polarization-plane to the left, and reducing alkaline solutions of copper after long boiling. More accurate investigations enabled him to identify these substances with acetone compounds.
Moritz recommends the following method of discovering whether we have to deal with grape-sugar, glycuronic acid compounds, or other (azone) crystal-forming substances.
Several litres of normal urine are precipitated with lead chloride and filtered; the filtrate is precipitated with ammonia and again filtered; the residue on the filter is washed and then dried on a clay slab. It is after this decomposed with oxalic acid, mixed with acetate of lead in excess, and the filtrate is deprived of its lead by hydrosulphuric acid. As a result is obtained a perfectly clear fluid to which the phenylhydrazin test described above is applied.
The precipitate obtained is filtered off, repeatedly washed with chloroform and alcohol, several times crystallized, and finally the melting-point of the needle-shaped crystals, which can be seen with the naked eye, determined. If sugar is present, the melting-point of the crystals will be at a temperature of 205° centigrade. If the melting-point lies below this temperature, we have to do with other substances (azones).
Hence it appears that we possess in the phenylhydrazin test, applied in the manner above described, a certain method of detecting even the faintest traces of sugar in the urine.
Although hitherto it has been often ascertained that sugar was present in normal urine, that was demonstrated only by the other methods with which we have been hitherto acquainted, and not by means of the phenylhydrazin test in the manner in which we have explained its use. This seems to have been the reason why different authors have not been able to speak unanimously on this subject.