205. CHARACTERS OF GLUCOSES. 217 



If this is the case, such a method cannot of course be employed 

 for the quantitative determination of the glucose. But if the solution 

 soon passes into a state of fermentation, and a tolerably large 

 quantity of gas is produced, an estimation of the amount of 

 glucose present may be made in the following manner: The 

 glucose solution is prepared as directed for the qualitative test, 

 and a measured quantity of it introduced into the flask A (Fig. 9). 

 The second flask B, shown in the figure, contains sulphuric acid, 

 as for the determination of carbonic acid according to the method 

 of Fresenius and Will. The whole apparatus is wiped dry, ac- 

 curately weighed, and exposed to a temperature of 20 to 30. 

 The carbonic acid evolved in A, passes through c, and is dried by 

 the sulphuric acid in B, before being expelled through d. After 

 the lapse of about two days, when the evolution of gas has ceased, 

 the carbonic acid in the apparatus is driven out by aspirating at d. 

 The difference in weight is the amount of carbonic acid produced, 

 100 parts of which are equivalent to 204*54 of glucose. 1 



205. Characters of the Various Glucoses. In speaking of glucoses 

 in the preceding paragraphs, I referred principally to grape-sugar, 

 fruit-sugar, and the combination of both known as invert-sugar. 

 Grape-sugar .(dextrose) possesses the following characters : it can 

 be obtained, by gradual deposition from solution, in prismatic 

 crystals 2 containing one molecule of water (cf. 89) ; an aqueous 

 solution, prepared fresh and without heat, in which the sugar is 

 present in the crystalline modification, shows a rotatory power for 

 (a) D + 91'81 (p = 1) ; whereas a solution that has been heated, in 

 which the sugar has passed into the amorphous condition, possesses 

 for (a) D a rotatory power of 49'54 to 50'00, and 46'34 3 (p = 12). 

 Hoppe Seyler 4 found the rotatory power of amorphous grape- 

 sugar to be (a) D = 56-4, from which he obtained the constant 

 quantity A'D = 1773'0; this may be used for calculating the 



1 Another method that has been suggested of estimating glucose consists 

 in determining the specific gravity before and after fermentation (for 20 to 24 

 hours) and calculating 0'219 per cent, of glucose for each O'OOl in the differ- 

 ence. Manasse'in has shown that the method yields good results with the 

 sugar in urine (Med. Centralblatt, 551, 1872). 



2 Similar crystalline deposits, especially sphserocrystals, have been occa- 

 sionally observed in the microscopic examination of dried drugs. Compare 

 Braun, Zeitschr. d. oesterr. Apoth. Ver. xvi. 337, 1878. 



3 Compare Hesse, Annal. d. Chem. und Pharm. clxxvi, 89, 1875 (Pharm. 

 Journ. and Trans. [3], vii. 191 et seq.). 



4 Zeitschr. f. anal. Chem. xiv. 303, 1875. 



