THE METHODS USED IN MAKING THE UNIT. 
I The glassware . — The glassware employed in making the immunity 
ij unit for standardizing toxines and antitoxins is used for no other pur- 
pose. To a certain extent the glassware used for measuring and 
I diluting the toxines is kept separate from that used for the serums. 
The pipettes, burettes, graduated flasks, and mixing cylinders have 
been carefully calibrated or standardized by the Bureau of Standards, 
i to whom we are under man}- obligations, not onh" for this work but 
for suggestions as to the most accurate methods of measuring precise 
amounts of fluids. 
I So far as possible all measurements are made with “capacity” 
I instruments. “Outflow” instruments, made “to deliver” given 
quantities, are only used in the measurements of the final dilutions, 
[I in which case it is evidently impossible to use capacity pipettes. 
' The glassware used for the purposes of this special work has been 
J calibrated with particularly fine graduations that pass halfwa}" or all 
j the way around the pipette, burette, or neck of the flask. 
’ The diluting fluid is measured in mixing cylinders of a particular 
[j design made for me by Messrs. F. & M. Lautenschlager, of Berlin, 
German}". Two of these mixing c}"linders are shown in fig. 9. We 
have a series of these cylinders, each measuring quantities in inter- 
vals of 10 c. c. up to 100 c. c. The first set of cylinders is graduated 
to contain from 0 to 10 c. c., the next size from 10 to 20 c. c., then 20 
to 30 c. c., 30 to 10 c. c., and final!}" 90 to 100 c. c. It will be noticed 
by the illustration that these mixing cylinders are subdivided into 
j tenths, which makes it possible to measure our diluting fluid with 
I great accuracy. ' It will also be seen that the cylinders have neither 
! neck nor shoulder, and that the ground-glass stopper fits directly into 
j the cylinder without a constriction. The internal diameter of these 
• cylinders is just 12 mm., which readily permits the introduction of 
any one of our pipettes. The smaller pipettes average 6 to 8 mm. in 
outside diameter; the larger ones do not exceed 10 mm. Attention is 
also invited to the fact that the shape of the cylinders readily allows 
thorough mixing and cleansing. 
We also use a style of mixing flask, shown in fig. 10, which was 
made for me by the Arthur H. Thomas Company, Philadelphia, Pa. 
The smaller flask is graduated to contain 19 and 50 c. c. ; the larger 
flask is graduated to contain 98, 99, and 100 c. c. As we have frequent 
occasion to use these quantities, the flasks are particularly convenient 
on account of their simplicity and comparative cheapness. It will be 
noticed that the neck has no shoulder, or constriction. 
( 55 ) 
