On Cutaneous Absorption, 295 



opaque, whitish appearance, after mixing the sulphat 

 with pale urine. 



EXPERIMENT IX. 



I accidentally discovered that potash' is an excellent 

 test for the presence of madder. A saturated solution, 

 in water, of caustic, or moderately carbonated potash, 

 dropped into a weak infusion of madder, turns it to a 

 bright cranberry-red, without destroying the transpa- 

 rency of the infusion. 



This test was applied to the red urine of the second 

 and fourth experiments, portions of which I had fortu- 

 nately preserved ; the same cranberry-hue was instantly 

 visible. 



I made a saturated infusion of madder in recently- 

 drawn pale urine. I dropped this saturated infusion, 

 into a quantity of pale urine, until it acquired precisely 

 the colour of the red urine of the fourth experiment. 

 The potash added to each, produced the same degree of 

 cranberry- redness. I should have applied this test to 

 the serum of the fourth experiment, but it had com- 

 menced a spontaneous decomposition. The serum of 

 the seventh experiment was not changed by the addi- 

 tion of the potash, in any other way than in becoming 

 paler in colour, in proportion to the degree it was di- 

 luted. 



