244 Wisconsin Academy of Sciences , Arts and Letters . 
of NaCl plus 10 grams of sugar in lOOcc., was suspended in a 
dish in 300 cc. of water. It was found that the sugar passed 
through the rind much more slowly than the salt. The experi¬ 
ment was continued for seven days. 
A similar experiment in which 80 cc. of an aqueous solution 
containing 10 grams sugar plus 10 grams urea in 100 cc. was 
placed in the upper half of the rind of a California grape 
fruit suspended in a dish containing 400 cc. water, yielded the 
result that both urea and sugar pass through the rind, but the 
latter more slowly. After twenty-four hours sugar may be de¬ 
tected in the outer liquid with Fehling’s solution. The urea 
acts on the rind thickening and hardening it. Much urea is 
thus retained in the rind, also considerable amounts of sugar. 
These facts were determined by an examination made after the 
experiment had run for seven days. 
To 70 cc. of a saturated solution of boric acid in water 10 
grams of sugar were added. This solution was placed in the 
upper half of the rind of a California grape fruit, the outside be¬ 
ing bathed by water. The experiment was continued for five 
days, the rind remaining intact and practically unaltered during 
this time. Only traces of boric acid passed through the rind. 
Sugar passed through slowly; but at the end of five days it was 
found in the outer liquid in considerable quantity. 
It was found further that in dilute aqueous solutions H 2 S0 4 , 
HC1, and HNO s readily pass through the skins of grape fruit; 
but the rinds are much altered by the acids, appearing shrunken 
and darkened in color. Citric acid passes through less rapidly 
than the mineral acids mentioned. It is apparently retained to 
a considerable extent in the rind. 
The experiments made with the rinds of grape fruit were 
also repeated with the skins of California oranges, with prac¬ 
tically the same results. 
QUANTITATIVE MEASUREMENTS OF OSMOTIC PRESSURES. 
The quantitative measurements of osmotic pressures were 
made with so-called semi-permeable membranes, that is to say 
with membranes through which the solvent* passes so much more 
readily than the solute, that the amount of the latter which 
