22 



and W. D. Halliburton 1 that the peptones are precipitated by tannic 

 acid. The last-named writer says " completely precipitated." In one or 

 two of the writer's own experiments, using tannic acid, an abundant pre- 

 cipitate was formed. This became clotted on heating and the clear 

 supernatant liquid showed some little return of turbidity 011 cooling. 

 The writer is inclined, however, to attribute this apparent partial re-so- 

 lution of the precipitate merely to the presence of a little of a proteose 

 formed in the earlier stages of digestion and not afterwards completely 

 removed. Assuming this view to be correct, tannic acid furnishes the 

 reagent needed to dispose of the one case unprovided for by phospho- 

 tungstic acid. 



DETAILS OF THE METHOD WITH PHOSPHO-TUNGSTIC ACID. 



The method proposed is as given in the following paragraphs. It is 

 stated, for the sake of simplicity, first, as applicable to meat, raw or 

 cooked. The variations required in the examination of other classes of 

 food materials are reserved for notice afterwards. 



A carefully selected and accurately weighed sample is to be ground 

 in a glazed porcelain mortar with as much sharp edged siliceous sand, 

 previously heated to redness with free exposure to air, or with as much 

 hard glass in small, sharp splinters similarly ignited, as shall suffice to 

 thoroughly subdivide the tissue and reduce it to the condition of a 

 smooth pulp. Of this pulp, very carefully mixed, so as to insure uni- 

 formity, two aliquot parts are to be taken. In one the total nitrogen is 

 to be determined by the well-known Kjeldahl process with addition of 

 potassium sulphate, as recommended by Gunning, using a rather large 

 proportion of sulphuric acid, so that no previous drying of the sample 

 is needed. The other part is to be digested with cold water, filtered on 

 a nitrogen-free filter, 2 and the residue washed on the filter with water 

 at the same low temperature as long as it gives up soluble matter in 

 sensible amount. Cold water is used to avoid action on and extraction 

 of the gelatinoids. Kreatinin is quite easily dissolved, as is also sarco- 

 sine; kreatin with a very fair degree of ease. Xanthin, hypoxanthin, 

 and carnin are less soluble. 3 



The filtrate is then to be slightly acidified with acetic acid, heated 

 to about 90 C., and again filtered from any coagulum produced. A 

 little more sand or pulverized glass may with advantage be stirred in 

 before bringing it onto the filter the second time. 



To this second filtrate is to be added an acidified solution of phospho- 

 tungstic acid as long as a precipitate continues to form, avoiding any 

 very large excess of the reagent solution. With a moderate amount of 



'Watt's Dictionary of Chemistry, revised edition, 1894, 4, 331. 



2 The ease with which nitration may be effected is much increased by the presence 

 of the sand or crushed glass. 



^Hypoxanthine, 1 part in 300 of water. The solubility of carnine does not seem 

 to have been recorded till now. The writer has found it to be 1 part in 312 of water 

 at 15.3- C. 



