1839.] 



On Chemical Tests. 



313 



makes it first whitish, and afterwards black, When the quantity 

 of iron is small, as in some mineral waters, a slice of gallnut may be 

 suspended in the liquid by a silk thread for two or three days. The 

 iron ought to be in the state of peroxide for the gall test to act imme- 

 diately. If it be in the -tate of protoxide, the test does not act till af- 

 ter some time. Gall tincture acts also on other metals. See the tables 

 of re-agents. 



a. By applying tincture of galls to a solution before and after boiling, 

 it may be known whether the iron is held in solution by carbonic, or 

 by a mineral acid. If the tincture acts before boiling and not after- 

 wards, carbonic acid is the solvent. If it acts both before and afier 

 boiling, a mineral acid is the solvent. If by the boiling, a yellowish 

 powder be precipitated, and yet the gall tincture continues afterwards 

 to discolour the solution, the iron, as often happens, is held in solution 

 by both carbonic, and a mineral acid. 



56. GELATINE, see jelly. 



57. GLUCINA is detected by the carbonated alkalies. See the ta- 

 bles of re-agents. Salts of glucina are not precipitated by oxalate of 

 ammonia, nor tartrate of potash, which distinguishes them from the 

 salts of yttria. The tests for alumina act in the same manner with glu- 

 cina as they do with alumina. See 26. 



a. Glucina is distinguished from alkaline and calcareous salts in the 

 same manner as alumina. It is distinguished from alumina by giving 

 no crystals of alum with potash and sulphuric acid; and by the colour 

 of its flame before the blow-pipe becoming dark grey, or black ; and not 

 blue, like alumina. 



b. Litmus paper is reddened by the neutral salts of glucina. 



c. Glucina, yttria, and zirconia occur in but few minerals, and in 

 small quantity. They are rarely met with. 



58. GliUTEN detected, see 1. 



59. GOLD is detected by sulphate of iron, or by sulphurous acid, 

 which produce in very dilute solutions first a blue colouring, and after- 

 wards a brown precipitate of metallic gold ; by oxalic acid which oc- 

 casions a greenish black colour, metallic gold afterwards subsiding ; and 

 by fresh made muriate or proto-chloride of tin which gives a purple 

 precipitate, see 122. Solutions of Gold may be distinguished by the 



