216 



GLUCOSES, SACCHAROSES, ETC. 



for this purpose heating for an hour to 60° with excess of alkaUne 

 copper solution, filtering off, and weighing the cuprous oxide pre- 

 cipitated (§ 83). The errors involved by this method, which at. 

 the best of times can jdeld only approximate results, have been 

 the subject of conimunications '^j Fresenius^ and Gratama.^ 



§ 204. Fermmtation Test (§ 86). — To detect glucose by this test 

 a little of the. suspected solution is evapomted until it contains at 

 least 5 per cent, of sugar. After cooling a few drops of tartaric 

 acid solution are added, and a little good yeast, previously washed 

 with distilled water ; 1-2 cc. of this mixture are then introduced 

 into a eudiometer standing over mercury. If glucose is present 



Fig. 9. 



an evolution of carbonic acid will soon begin, and should, in a few 

 hours, displace a considerable amount of mercury (§61). By 

 means of a blank : experiment with distilled water, and the same 

 quantity of yeast, it may be ascertained whether any carbonic acid 

 has been produced by the yeast itself alone. 



The absence of any evolution of carbonic acid is not necessarily 

 a proof that the liquid is free from glucose. Certain constituents 

 of plants, such as salicylic acid, thymol, etc., may occasionally be 

 present, and prevent any fermentation taking place.* 



' Anieit. zur quant. Analyst;, 6th editionj 833. 



2 Zeitschr. f. anal. Chem. xvii. 1875, 1878 (Journ. Chem. Soc. xxxiv. 611), 

 ^ Compare Wernoke, 'tXeber die Wirkung einiger Antiseptioa auf Hefe.' 

 Diss. Dorpat, 1879. 



