288 



then to remove any turbid substances by means of filtering, before 

 this i-eagent is nsed. 



Protochloride of tin [sub-chloride of tin] acts as precipitating 

 agent similar to acetate of lead, but a great disadvantage is 

 that it must be employed in a very acid solution, and even then 

 soon becomes oxydised. It deserves, thei-efore, no recommenda- 

 tion. 



Sulphate of copper [blue vitriol] (1 j^art in 9 parts water) yields, 

 with a few organic acids, pi-ecipitates of a characteristic colour, 

 for instance, a green one with salicylous acid ; but may be easily 

 dispensed with. 



Alkaline tartarate of copper is used as a direct test for grape-sugar 

 (glucose) or fruit-sugar, also for the direct quantitative estimation 

 of grape-sugar and fruit-sugar, and for the indirect one of cane- 

 sugar and starch. To prepare it, dissolve 3-465 grams pure 

 crystallised sulphate of copper in about 40 grams water, add 

 9 grams pui-e tartaric acid, and, after it is dissolved, 18 grams 

 hydrate of soda and dilute, after this is also dissolved, with so much 

 water that the whole amounts to 100 cubic centimeters. Ten C. C. 

 of this deep-blue liquid, containing 0'3465 grams sulphate of 

 copper, require for the reduction of the oxyd to sub-oxyd 0'05 

 grams grape or fruit sugar. 



Cane-sugar and starch miist be previously converted into gi-ape- 

 sugar by digestion with diluted sulphuric acid. 500 parts grape- 

 sugar, as found above, are equal to 475 parts cane-sugar, or to 

 450 parts starch. 



Liquids which have to be tested with this reagent must be 

 alkaline, or at least neutral ; acid liquids have therefore first to be 

 neutralised by potash or soda-ley. 



This i-eagent, when kept ready prepared, must be tried every 

 time before using it on its unimpaired quality by heating it to 

 the boiling point, when it must become neither discoloured nor 

 turbid. 



Tannic acid [gallotannic acid], (1 part in 9 parts alcohol of 50 per 

 cent. ; for precipitating purposes : 1 part in 9 parts water, prepared 

 shortly before vise). An impoi-tant precipitating agent of alkaloids 

 and of many indifierent bitter substances. As a rule these pre- 

 cipitates do not allow washing with jjure water without being 

 decomposed. In order to isolate the substance combined with the 

 tannic adid, the precipitate is spread out on several thicknesses of 

 blotting prper,, supported by a tile or bi-ick, it is then mixed with 

 oxyd of lead or with white of lead and eventually with a little 

 water, is dried at a gentle heat and extracted with alcohol, which 

 leaves undissolved the tannic acid in combination with lead. 



Bi-iodide of jmtassium (4 parts iodide of potassium and 3 parts 

 iodine in 93 parts water). Serves principally to distinguish 

 starch, to which it imparts a violet or deep-bhie colour. It may 



