314 PROTECTIVE INOCULATIONS. 
his conclusion that for producing immunity in man, one hundred and 
fifty normal antitoxin units should be given, instead of sixty as he 
had previously recommended. 
The serum manufactured under his direction is said (September, 
1894) to be of two kinds—one, obtained from the horse, has a value 
of sixty normal antitoxin units; the other has a value of one hundred 
and forty units. Of the weaker serum Behring says experience has 
demonstrated that for children under ten years of age ten cubic cen- 
timetres is sufficient to arrest the progress of the disease and effect a 
cure if given within two or three days from the outset of the attack. 
For producing immunity in children subject to infection, one-tenth 
of this amount (one cubic centimetre) is said to be sufficient. Of 
the stronger serum one cubic centimetre is sufficient to arrest the dis- 
ease during the incubation period; and, according to Behring, out of 
one hundred cases treated during the first forty-eight hours with a 
single therapeutic dose (ten cubic centimetres of serum having a value 
of sixty, equals six hundred normal units), not five will die. The 
later the treatment is commenced the larger will be the dose required. 
Behring further states that the diphtheria antitoxin has no injurious 
effect upon animals in the largest doses that have been employed, 
and that aside from its antitoxic power its properties are entirely 
negative so far as living animals are concerned. 
Aronson (1893), in experiments on dogs, succeeded in producing 
immunity by the use of attenuated cultures, or of cultures to which 
formaldehyde had been added; also by feeding the animal large quan- 
tities of diphtheria bouillon; and, finally, by injection of the blood of 
naturally immune animals (white rats) into which large quantities (ten 
cubic centimetres) of a virulent culture had been injected. Two months 
after receiving several such injections it was found that 0.2 gramme 
of blood-serum from the rat sufficed to save a guinea-pig from fatal 
infection. In experiments on dogs an immunity was established in 
six weeks by the injection of a large amount of a virulent culture. Its 
serum had a value of 1: 30,000, 7.e., 0.01 cubic centimetre of this serum 
sufficed to protect a guinea-pig weighing three hundred grammes. 
From one hundred grammes of this serum Aronson claims to have ob- 
tained 0.8 gramme of a substance which had a value of 1:500,000, as 
tested in the treatment of an animal which had received ten times 
the minimum fatal dose of a two-days’ bouillon culture. A ten-per- 
cent solution of this substance had, therefore, ten times the value of 
Behring’s “normal serum.” The precipitated antitoxin was soluble in 
water, and more readily so in a slightly alkaline solution, and gave 
all the reactions of an albuminous body. When dried in vacuo at 
