Structure and XJses of the Spleen . 51 
The cells of the spleen were more distinctly seen than in the 
former experiment, particularly at the great end. 
Although there was every reason to believe that the colouring 
matter of the madder had been conveyed into the urinary 
bladder, yet so muddy and indistinct was the colour, that it 
was by no means completely ascertained. I therefore resolved 
in my future experiments, to make use of some colouring 
substance, the presence of which could be detected in a very 
diluted state, by means of a chemical test ; and I requested 
Mr. W. Brande, of whose assistance I have before availed 
myself, to point out the substances best fitted for this purpose. 
He immediately suggested that rhubarb was a substance which 
he had made use of as a test to ascertain the presence of alkali, 
and therefore had no doubt that the caustic alkali would prove 
a test of rhubarb. This substance has also another advantage ; 
it is well known to pass very readily by the kidneys, without 
being decomposed. 
The following are the results of experiments made with 
rhubarb, to ascertain the best modes of detecting it in the urine 
and blood, and the time it takes to pass from the stomach to 
the urinary bladder. 
Five drops of tincture of rhubarb added to 3 ounces of 
water, are found to strike an orange tint when the test is 
added, which does not take place when the rhubarb is more 
diluted. 
Six drops of tincture of rhubarb, added to three ounces of 
serum, are readily detected by the eye, but the colour is not 
heightened by applying the test ; the alkali contained in the 
serum, being sufficient to strike as bright a tint, as that quan- 
tity of rhubarb can receive from the addition of alkali. 
H 2 
