432 PEACTICAL PHYSIOLOGY 



stance to liberate its sulphur, NaOH prepared from metallic sodium must be 

 used, because ordinary NaOH contains S as an impurity. 



The phosphorus is precipitated by adding about a fourth its bulk of 

 a solution of ammonia molybdate containing nitric acid, and a fourth 

 its bulk of a saturated solution of ammonium nitrate. The mixture is 

 placed on the water bath at about 50 C., when a yellow precipitate 

 of molybdenum phosphate will develop. If it be desired to quanti- 

 tatively determine the amount of phosphorus present, this precipitate 

 is collected on a filter paper, washed with a weak solution of am- 

 monium nitrate, and then dissolved in a 3 per cent, solution of ammonia. 

 The phosphorus is now precipitated from this solution by the addition 

 of magnesia mixture, the resulting precipitate being allowed to stand 

 overnight, and then collected on an ash-free filter paper, and incinerated 

 in a crucible. The ash, which consists of Mg 2 P 2 7 , is weighed, and 

 from it the amount of phosphorus calculated. 



Nucleic Acid. If nuclein be decomposed by means of alkali, the 

 albumin is split off and the nucleic acid combines with the alkali to 

 form a salt, which can then be decomposed by the addition of acid 

 alcohol, when the nucleic acid will separate out as a precipitate. 



Mix 100 grammes of baker's yeast with 300 c.c. of a 3 per cent, 

 sodium hydrate solution, and thoroughly stir the mixture for about 5 

 minutes. Now add weak hydrochloric acid till the solution is 

 neutral, and then acetic acid till no more precipitate of the proteid 

 results. Filter off the proteid and add water to the filtrate to make its 

 volume equal to 300 c.c. Then add 5 c.c. of a 20 per cent, solution of 

 hydrochloric acid so as to obtain, approximately, a mixture containing 

 0-4 per cent. HCL Now add an equal bulk of spirit containing 0'4 per 

 cent. HC1, when the nucleic acid will be precipitated. It is collected 

 on a filter paper, and if it be desired to still further purify it is 

 dissolved in weak ammonia water and again separated from proteid, 

 as above described. 



Chemically, nucleic acid consists of a compound of phosphoric acid, and 

 alloxuric or nuclein bases (for the chemistry of these see p. 179). The 

 phosphoric acid can be detected by the method described under nuclein. 

 In order to isolate the nuclein bases the nucleic acid, or organ containing 

 it, is boiled with 0-5 per cent, sulphuric acid for four hours ; filtered ; 

 the filtrate treated with acetate of lead to separate the proteid; the 

 lead removed by means of sulphuretted hydrogen, and the bases pre- 

 cipitated in the lead-free filtrate by means of an ammoniacal solution 

 of silver nitrate. 



The nucleic acids separated from different organs and tissues yield 

 different bases, and they are sometimes classified according to the nature 



