54 
Dissolving the dried serum . — Once every two months, or whenever 
it is desired to test the serum, one of these bulbs is broken and 1 gram 
of the serum carefully weig'hed and dissolved in ph^-siological salt 
solution (0.85) one part, Price’s gdycerin two parts. 
Testing . — This glycerinated solution of blood serum is now accu- 
rately standardized in order to determine its precise antitoxic value in 
terms of Ehrlich’s immunity unit. 
The number of immunit}^ units contained in a given quantity of the 
serum is determined by the test (LT-) dose of toxine. 
That quantity- of serum necessar}" to add to the L+ dose of the 
toxine so that the mixture will kill the guinea pig on about the fourth 
dav contains just one immunity unit. In the commercial testing of 
serums in order to have a coefficient of safety it would be safer to 
require the pig to live. 
We use a number of toxines, the L+ doses of which have been very 
painstakinglv ascertained on a large number of animals. The details 
of the technique bv which this part of the testing is carried out will 
be found under the caption “Methods used in making the unit,” and 
need not be repeated here. 
