PREFACE. 
The object of this bulletin is to describe the methods b}" which the 
immunity unit for measuring the strength of diphtheria antitoxin is 
obtained, and the principles involved. 
The unit is based on the one established by Ehrlich, and has been 
made by comparison with the normal serum sent to this laboratory 
by the Kgl. Pr. Institut fiir experimentelle Therapie, Frankfurt a. 
M., Germany. 
On account of the technical difficulties met in testing the strength 
of diphtheria antitoxin, and the intricate composition of the diphtheria 
poison, as w^ell as the peculiar chemical relations existing between the 
toxine and the antitoxin, it has been necessaiw to enter into the many 
details of every portion of the process in order that those who use 
this standard may do so under the same conditions. Otherwise com- 
parable results would not be obtained. 
In such an apparent!}" simple procedure as measuring two pieces of 
metal, as is done in duplicating the standard meter and yard, the 
greatest attention to many details must be observed. It can there- 
fore be well understood how important it is, in comparing a standard 
based upon the physiological action of a mixture of two complicated 
organic substances, not to neglect the smallest detail that would insure 
accuracy. 
As the unit made in the Hygienic Laboratory, U. 8. Public Health 
and Marine-Hospital Service, is the legal unit for this country, the 
methods by which it is produced, as described in this bulletin, may be 
taken as official. 
All standards are fundamentally arbitrary quantities. If all the 
meter bars or yardsticks w^ere lost it would be difficult if not impos- 
sible to produce others having the exact lengths of the originals, which 
are therefore guarded against harm under conditions of uniform 
temperature, etc., to prevent changes in length. 
The antidiphtheritic serum that has been carefully standardized and 
is used as the unit of strength for the measurement of diphtheria 
antitoxin is similarly kept under strict conditions of light, heat, mois- 
ture, etc., in the Hygienic Laboratory, U. S. Public Health and 
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