312 NATURE AND DETECTION OF METALLIC POISONS. 



and it was not till then, that I became acquainted with the cu- 

 rious lacts above mentioned. It immediately occurred to me, 

 that gallate of lead was insoluble ; and I lost no time in mak- 

 ing some experiments, to ascertain the fact. The method by 

 which I prepared some gallic acid for the purpose may, per- 

 haps, be new, and not wholly uninteresting, to some of your 

 Mode of pre- readers. My first step was to make a strong tincture of nat- 

 -acid"^ ^^""^ sails in proof spirit. To this was added, by little at a time, 

 a nearly saturated solution of isinglass, till the whole of the 

 tannin was precipitated. The liquor separated from the coa- 

 gulum at first appeared rather opaque, but without colour. 

 By standing at rest for a few days, a deposition of flocculent 

 matter took place, consisting of gelatine and tannin, which left 

 the liquor transparent and colourless. This I considered as a 

 solution of gallic acid, nearly pure. At all events, it did not 

 contain any substance which prevented its being an excellent 

 A sensible test test for iron or lead, I soon found, that in very dilute solutions 

 of lead, Qf le^d, where sulphuric acid, or a sulphate, produces no visible 



precipitate, the presence of this metal was made sensible by the 

 aid of, the gallic acid. This confirmed my suspicions on the 

 subject, and left me in no doubt as to the real cause of the rum 

 losing its pernicious qualities ; for, since the joint existence of 

 lead and gallic acid in any fluid is impossible, from the forma- 

 tion of an insoluble gallate, the lead of the rasa becomes 

 precipitated by the gallic acid furnished by the oak cask. 

 Means of de- These facts supply an excellent, though indirect, method of 

 ^•jjp ft ^ '° ascertaining, in many instances, whether lead be dissolved in 

 wines. If, on testing the wine with iron, it is found to contain 

 gallic acid, we may safely infer, that no lead is present j but if 

 ' no gallic acid be detected, then either this acid, or the sul- 

 phuric, may be added, which will precipitate the lead in the 

 form of a white powder. Sulphuret of potash, or lime, may 

 also be employed, which will occasion a blackish deposit. 

 Lead should The prevailing use of lead in dairies is very objectionable, 

 not be used in especially when the milk is immediately used as an article of 

 food. On the separation of the curd and butter, the dissolved 

 lead will, no doubt, exist in the whey. When milk is kept too 

 long in warm weather, the acetic acid is formed, which takes 

 up the lead ; and it is a fact well known in dairies, that milk 

 remains sweet longer in leaden vessels than any other. This 



is. 



