439 
Use of Collodion Osmometers. 
most biological purposes the former is sufficient, as the osmotic pressure of 
a biological fluid may be determined in terms of a standard substance such 
as cane sugar, and thereby obtained either from tables or from theoretical 
calculation. For the absolute measurement of osmotic pressure (except 
where it is very small, as in the case of colloidal solutions) these osmo- 
meters cannot be used, as they stretch under moderate pressure. Semi- 
permeable collodion membranes in sheet form could, however, be so used, 
but some type of apparatus to take up the pressure developed would 
obviously be necessary. 
These osmometers, as far as is known, last indefinitely. If kept in 
water they tend to become coated with fungoid growths so that they should 
be kept in a dilute solution of an antiseptic, e. g. dilute copper sulphate. 
