Urea possesses neither an acid nor alkaline reaction. Its crystalline form is that of a four-sided prism.
Extractive matters of Urine. — There are four of these extractives ; one soluble in alcohol as well as in water, and three soluble in water, and not in alcohol.
That which is soluble in alcohol may be ob tained from the urine by digesting alcohol of specific gravity 0.833 on an extract of urine, and after crystallising the urea by means of nitric acid from the products of the alcoholic solution dissolved in water, separating the un crystallisable matter, and neutralising it with carbonate of baryta : the mass must then be dried, and alcohol must be used to separate the extractive from the barytic salt.
Its chemical properties are as follow : — When heated, it swells much, and leaves a copious alkaline carbonaceous mass.* It red dens litmus paper.
Neither bi-chloride of mercury, nor the acetate of lead, is capable of precipitating its watery solution.t Both acid and alkaline solutions are inca pable of effecting any precipitation of this ex tract from its solution in water.
Protochloride of tin, nitrate of silver, and di-acetate of lead, produce precipitates.
It may be well to mention, that if anhy drous alcohol be digested on this extractive, which has been called osmazome, it is capable of being divided into two portions; the one soluble and the other insoluble in that fluid.
The property of being precipitated by the di-acetate of lead, nitrate of silver, and pro tochloride of tin, belongs peculiarly to that part of the extractive matter which is soluble in anhydrous alcohol.
Animal extractive soluble in water only. — This may be procured by dissolving in water an extract of urine, which has been digested with alcohol of specific gravity 0-833. By the re-solution we separate any vesical mucus, lithic acid, earthy phosphate, or silica, which may be cditained in the mass. The solution is now precipitated with acetate of in order to rid it of sulphuric acid. The sulphate of baryta is collected on a filter, and the filtered liquor neutralised with ammonia, and then again thrown down with the acetate, which now causes a precipitate of phosphate of baryta.* This is to be collected, and the
filtered liquor evaporated in order to drive off the ammonia ; or, what is better, it may be neutralised by acetic acid. Neutral acetate of lead is now added to the solution, which causes a copious precipitate. This must be collected and washed, and then decom posed by sulphuretted hydrogen, which pre cipitates sulphuret of lead, and leaves the animal extractive in solution. This may be obtained by evaporation. This extractive is, however, but part of that meant to be under stood as the "animal extractive soluble in water only," so often mentioned in analyses. The remainder of it may be procured by pre cipitating the liquor (in which the precipitate by neutral acetate of lead subsides), by means of the di-acetate of lead ; then collecting the precipitate, as before by sul phuretted hydrogen, and procuring the ex tractive from the clear liquor. It must be re membered that each of these extractives has peculiar properties, perhaps dependent on the processes used to obtain them. There is also a portion of animal extractive left unpre cipitated by the di-acetate of lead. It is easily obtained from the liquor by ridding the solution of any lead which may exist in it by means of sulphuretted hydrogen, filtering, and then evaporating to dryness.
It is a mixture of these three peculiar ex tractives which constitutes the "animal ex tractive soluble in water only" of Berzelius.
The properties of the extractive matter pre cipitated by the neutral acetate of lead are as follows : It is of a brownish colour, translucid, and does not deliquesce; has no taste, and scarcely affects litmus paper.
Its solution is rendered cloudy by corrosive sublimate, and more so by the protochloride of tin.
The extractive precipitable by the di-ace tate of lead has the following properties : It is of a yellowish brown colour ; it has a slightly bitter taste, and does not deliquesce.