798 
gous serum and is easily able to destroy a small quantity in a few 
days, even if the titre is very high. 
To obtain an immunity for a longer time, a quantity of serum has 
to be injected, which the organism, even if it defends itself vigo- 
rously against the foreign serum, cannot destroy too soon. The 
disadvantage that is attached to the injection of a large quantity of 
serum, namely the developing of symptoms of serum disease, which 
are always temporary, is not of great importance when a perilous 
illness is to be prevented. 
Relating to the passivei mmunization of the wounded against tetanus, 
which gave rise to our research, we came therefore to the conclu- 
sion that there is absolutely no cause to use for this purpose, as 
now commonly happens in our country, an antitetanus serum that 
contains in one c.c. 4 A.U. 
The injection of 10 ¢.c. antitetanus serum with a titre of 2 A.U. 
deserves to be preferred, because in this way, an equal degree of 
immunity is produced as by injection of 5 ¢.c. antitetanus serum of 
4 A.U., and the immunity lasts longer. 
Moreover, the results of our experiments give an important indication 
concerning the immunization against diphtheria. Years ago, when in 
all countries diphtheria serum was used with a titre of about 100 A.U., 
it has been fixed empirically that the injection of 5 ec.c. serum 
(= 500 A.U.) was sufficient to protect a child against diphtheria for 
about 3—4 weeks. Afterwards in some countries the titre of the 
diphtheria serum has been raised more and more. If now, — relying 
on the false supposition, that the duration of the immunity has 
nothing to do with the quantity of serum that is inoculated —, to 
prevent diphtheria, 1 ¢.c. diphtheria serum with a titre of 500 A.U. 
is injected into a child, expecting to get in this way the same result 
as formerly with the injection of 5 c.c. with a titre of 100 A.U., 
there is a great chance that the immunity, instead of 3 or + weeks, 
only lasts 1 week. 
Chemistry. — “On the Allotropy of the Ammonium Halides”, HI *). 
By Dr. F. E. C. Senerrer. (Communicated by Prof. P. Zeeman.) 
(Communicated in the meeting of Nov. 25, 1916). 
$ 14. In $ 1 I said that in the older literature statements occur 
which point to the occurrence of two different modifications of am- 
monium bromide and ammonium chloride, and that it has been 
demonstrated in a paper by Warracr that ammonium bromide is 
First paper. These Proc. XVIII p. 446. Second paper. These Proc. XVIII p. 1498. 
