309 



Suspend in a glass-jar the other half of the washed precipitate 

 in about five times its volume of absolute alcohol, treat with 

 sulphuret of hydrogen in excess, allow the latter to evaporate at 

 the open air, filter ofi" the sulphide of lead, and evaporate the 

 liquid with only a gentle heat, and at last in the vacuum. After 

 the alcohol has been driven ofl', dilute with water half of the 

 remaining liquid, to be used for testing purposes, and evaporate 

 the other half to dryness. 



The dried remnant, if consisting only of a yellowish varnish of 

 a pure, astringent taste (often followed by a slightly bitter flavour), 

 is most likely only a tannic acid, which then has to be charac- 

 terised in its aqueous solution by the proper tests of the preced- 

 ing paragraph (chloride of iron, glue, tartarated antimony). The 

 dry lead-compound serves for the elementary analysis and for the 

 estimation of the atomic weight. 



In the absence of tannic acids, the dried body aj^pears often in 

 crystals and with a more or less acid taste, and has then to be 

 tested on citric acid, &c., by means of lime-water. Acids of a 

 peculiar odour, as benzoic acid, cinnamic acid, &c., betray them- 

 selves to some extent by this jiroperty. 



A taste, at once astringent and acid, and a not entirely amor- 

 phous state, indicate a mixture of tannic with one or more acids 

 of a diflerent kind. A thorough examination of the latter is only 

 possible, after the tannic acid has been removed. This is done 

 most efiectually and without fear of contamination by cutting 

 isinglass into short, narrow strips, leaving these to soak in a 

 porcelain-dish with water, until converted into a swollen-up jelly- 

 like mass, adding the acid liquid in question and keeping the 

 whole at ordinary temperature and under stirring, until a sample 

 of the liquid on examination proves free from tannic acid. The 

 complete absorption of the tannic acid by isinglass is efiected 

 slowly and may take several days. Heat must be avoided entii-ely, 

 as likely to convert the glue into its soluble modification. After 

 the process is finished, the liquid has to be filtered, and containis 

 now the other acid or acids, the nature of v/hich has to be 

 examined. If necessary, the dry part of the lead-precipitate is 

 also decomposed Ln the same way, in order to obtain more of the 

 acid, and this ought to be done in all cases where two or more 

 reqiiire investigation. 



2. The portion of the lead-precipitate which remains undissolved 

 after the successive treatment with hot water and acetic acid (or 

 the whole lead-precipitate, if not afiected by hot water or by 

 acetic acid) may contain of mineral acids, phosphoric or svilphuric; 

 of organic acids, oxalic acid, and appears, as a rule, of a grey-brown 

 colour, from traces of hiimus-like or of other colouring matters. 



To test on oxalic acid, heat the moist precipitate to the boiling- 

 point with a solvition of carbonate of soda, boil for a quarter to 



