growth for this purpose.

2. At the end of this time the yeasts are killed by plunging the tube in
water heated to 80 C. and maintained at this temperature for fifteen
minutes. The contents of the tubes are then poured into a Hopkins
centrifuge tube which has a capillary tip graduated in hundredths of a
cubic centimeter. After twenty minutes centrifugating at a speed of about
2400 revolutions per minute the yeasts in the solution have all been
packed into the tip and the volume can then be read accurately to
thousandths of a cubic centimeter (with the aid of a scale and magnifier).
With a control tube containing 9 cc. of the sterile media and 1 cc. of
distilled water in place of the 1 cc. of extract a comparison can be
obtained which is an accurate measure of the stimulatory effect of the
extract. If this stimulus is due purely to vitamine it is obvious that
this procedure would enable us to compare extracts of known weights of and
arrive at comparisons which would be measures of their vitamine content.
In other words the procedure is now in a satisfactory form for testing and
its value depends merely upon our ability to show that the stimulus given
the yeast is due solely to vitamine "B."

The interest of the vitamine student in this test will be easily
understood for it is so simple of manipulation and so rapid in producing
results that it is the nearest approach to a chemical test of satisfactory
nature yet proposed but unfortunately evidence soon began to accumulate to
show that the stimulation produced by extracts of various sources is not a
matter of pure vitamine. If we plot a curve of stimulation for various
dilutions of a given extract we find that the stimulation is not directly
proportional to the concentration of vitamine present but is a composite
of several factors. The chart derived from experiments by Eddy and
Stevenson shows the general nature of this curve. Other experimenters have