404 Dr. J. Mellanby. [June 3, 
There is some evidence from the form of the curves (which may be drawn 
‘from the above figures) that the presence of diphtheria antitoxin does affect 
the precipitation values. Thus the Curve A is slightly concave to the 
left, and Curve D is slightly convex to the left. Curves B and C show 
features intermediate between A and D. The form of that part of the 
precipitation curve is affected in which the antitoxin is precipitated, and 
it may be seen that the degree of variation is roughly proportional to the 
amount of antitoxin present. : 
From this we may assume that although diphtheria antitoxin has the 
same alcohol precipitation properties as the great bulk of the proteins of 
normal serum, yet it is something added, and not merely a normal protein 
with an antitoxic group attached to it. 
Alcohol as a Protein Coagulant. 
An attempt was made to differentiate diphtheria antitoxin from the normal 
proteins of serum by means of alcohol as a protein coagulant. 
In the case of diphtheria antitoxin it was found that although alcohol 
added to serum did not affect its antitoxic value so long as no protein was 
precipitated, yet the rate at which precipitated antitoxin was destroyed by 
alcohol was the same as that at which the normal proteins of serum were 
coagulated. Therefore, so far as the coagulating action of alcohol was 
concerned, there was no evidence of any physical differentiation of diphtheria 
antitoxin from the proteins of normal serum. 
Precipitation by Alcohol below the Critical Potnt. 
The above results were obtained by the action of alcohol on diphtheria 
antitoxic serum at temperatures above the critical point. 
The alcohol precipitation limits of diphtheria antitoxin from serum were 
obtained at temperatures below the critical point. As a result of a series of 
alcohol precipitation experiments at 2° C., it was found that the greater part 
of the antitoxin was precipitated with other proteins of serum between 
16 and 28 per cent. alcohol. These antitoxin limits, so far as the quantity 
of precipitated protein is concerned, are practically the same as those 
obtained at temperatures above the critical point. The only difference 
between precipitation above and below the critical point was that in the 
former case a portion of the antitoxin was lost owing to the coagulating 
power of the alcohol, whilst in the latter case the total protein precipitate 
was soluble, and the whole of the antitoxin could be recovered. 
(B) Ammoniwin Sulphate-—Brodie precipitated the antitoxin from horse 
