396 Mr. Hefiry's Confiderations on different Materials, 



earthy or metallic falts, and then thrown into 

 the liquors impregnated with the colouring fub- 

 ftances, the colouring matter quits the other 

 principles to which it was united, feizes on the 

 earthy bafis of the fait, and uniting with it, 

 lofes its folubility in water, and, in this com- 

 bination, becomes attached to the material, in 

 fp permanent a manner, as not to be wafhed 

 away by water. M. Macquer, however, does 

 not feem to have been aware that it is by means 

 of the aftringent principle that this precipitation 

 is efFcdled. All the fubftances which form la- 

 ques contain this principle; as is evinced by the 

 blacknefs they produce with martial folutions; 

 and to this the colouring principle feems to be 

 clofely united. A few drops of infufion of galls 

 produces an immediate precipitation of the earth 

 of alum from its acid. This precipitation is 

 more copious than that produced by any of 

 the common colouring fubftances, and is, at 

 the fame time, white. 



Here again new experiments are fuggefted. 

 Let a large quantity of earth of alum be thus 

 precipitated by galls. Let the precipitate be 

 well walhed, and afterwards expofed in a retort, 

 ■with a receiver adapted to it, to a ftrong heat. 

 The aftringent principle, if united to the earth 

 of alum, being volatile, will probably be driven 

 over into the receiver, and thus the fuppofed 

 combination be rendered evident. 



Again, 



