23 
I Ehrlich ^ now made numerous researches in order to determine the 
I stability of the antitoxin. He had particular success with a serum 
I containing a high percentage of glycerin. This glycerinated serum 
showed exactly the same power of neutralizing toxine after one j^ear 
that it did at first. 
For a while this glycerin solution of antitoxic serum was used as the 
basis for standardizing serums. But Ehrlich showed that even glyc- 
erinated serum may weaken and also the fact that because the neu- 
tralization point does not vary is no guaranty of the stability of the 
test solution. It was therefore necessary to abandon the glycerinated 
serum as a method for preserving the standard unit. The principal 
factors which cause the weakening of antitoxin are moisture, oxygen, 
light, and heat. 
It is very easy to guard against the deleterious influences of light 
and heat. It was therefore necessary only to pay particular attention 
to the first two agents. 
The serum was therefore reduced to a powder under proper precau- 
tions and this dried blood serum preserved in a small glass apparatus 
consisting of two parts connected b}" means of a glass tube. The 
■ serum is placed in one portion of the apparatus and phosphoric anh}"- 
drid, which is a very powerful dehydrating substance, in the other. 
The air is exhausted as much as possible by means of a high vacuum 
and the apparatus hermetically sealed. After a few days the acid will 
have taken up all the moisture from the serum, and the tube connect- 
ing the two parts may then be melted and sealed ofi', separating the 
acid from the serum. The dried serum is now contained in a vacuum 
tube and preserved in a cool place in absolute darkness. 
In this way Ehrlich prepared a great number of these little tubes, 
each one containing a dried serum whose value was accurately deter- 
mined. Each tube contained 2 grams of the dried serum, repre- 
senting 1,700 immunity units in each gram. Every 'two or three 
months one of these tubes is carefully opened and the contents dis- 
solved in 200 c. c. of a 10 per cent solution of sodium chlorid and gl^^c- 
erin mixture, the mixture containing from 50 to 80 per cent of 
glycerin. One c. c. of this glycerinated serum represents then exactl}^ 
seventeen times the normal strength; and therefore if we dilute 1 c. c. 
of this glycerinated serum with 16 parts of water, each cubic centi- 
meter of the dilution contains one immunity unit. 
Although the serum is used as a basis for standardizing the strengths 
of other sera, the toxine still plays a very important role in the process 
of testing, as it is manifestly impossible to estimate the strength of an 
« Ehrlich: Die Wertbemessung des Diphtherieheilseriims imd deren theoretisclie 
Grundlagen. Klin. Jahrb., Jena, v. 6 (2), 1897, pp. 299-326. 
