54 
Dissolving the dried serum . — Once ever}^ two months, or whenever 
it is desired to test the serum, one of these bulbs is broken and 1 gram 
of the serum carefully weighed and dissolved in physiological salt 
solution (0.85) one part, Price’s glycerin 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 immunit}" unit. 
The number of immunit}" units contained in a given quantity of the 
serum is determined by the test (L4-) dose of toxine. 
That quantity of serum necessary to add to the L-f dose of the 
toxine so that the mixture will kill the guinea pig on about the fourth 
day contains just one immunit}" unit. In the commercial testing of 
serums in order to have a coefficient of safet}" it would be safer to 
require the pig to live. 
AY e use a number of toxines, the L+ doses of which have been veiy 
painstakingh" ascertained on a large number of animals. The details 
of the technique by Avhich this part of the testing is carried out will 
be found under the caption “YIethods used in making the unit,” and 
need not be repeated here. 
