On December 11 spores in normal gelatine were put in at 22° at 1.45, and into measurement at 6.15 P.M.

The normal doubling period :

20 25-41 μ = 6.45 P.M.-9.2 P.M. = 137 minutes at 145-15 1° C.

It is a very long and tedious business to work out these slow growths at low temperatures, but several other curves show that about 200 minutes represents the doubling period for 14° C.; the time rises very quickly as the temperature falls, however, and I find it is nearly 400 minutes for 10-12° C., and probably nearly 800 to 1000 minutes for 6-8°. This is, no doubt, partly due to the stiffness of the gelatine.

On December 10 spores were sown in normal gelatine at 22° C. at 10.30, and put into measurement at 3.30.

The doubling periods are (normal) as follows:

415 83 μμ = 40 P.M.-5.21 P.M. = 81 minutes at 17° C.

This shows very clearly the steady growth and equal doubling periods at constant temperature, and, at the same time, how slight an alteration in the temperature (causing variation either way) suffices to slow the growth.

Another segment in the same drop was now selected, and measured as follows:

Here the doubling period was :—

8.33 P.M.-9.49 P.M.76 minutes at 18-18 ∙1—17 ·5° C.

On November 17 spores sown as before, remained at 22° C., from 10.30 A.M. till 3.45, and were then put in at 19.5° C.

Doubling period :—

81.0-162 μ 162.0-324,,

= 4.5 P.M.-5.16 P.M. = = 71 minutes at 19·8-19 6-19 7-19 3° C. = 5.16 193-1875-19.5° C.

-6.26

= 70

Calculated back:

459 0-229 ⚫5 = μ 5.49 P.M.-7.11 P.M. = 82 minutes at 19-18 75-19.5° C. 2295-114 5,,

=

4.37 -5.49

- 72

Normal doubling period :

19.7-19° C.

910-1820 μ=4.15 P.M.-5.26 P.M.=71 m. at 19 7-19 6-19 7-19.6—19.3°C. 182.0-364 0,,=5.26,, -6.41 193-1875-19.5° C.

=75

On December 8 spores were sown in normal gelatine at 11 A.M., and put into the incubator at 22° C. till 2.30 P.M.

On comparing these periods with those of November 17, it will be seen how admirably the agreement comes out, and further shows that

the food-material has not degenerated in quality during the interval of nearly a month, for it had remained in the flask for that interval. From 6.15 onwards the above filament was segmented into a short and a long segment; in the following table I resume the further growth of the shorter one.

At 8.15 another segment in the same drop was chosen, and measured as follows:

:

Some interesting comparisons are possible here. For instance, the original filament was 65 μ long at 4.3 P.M., and it was 109 μ long at 4.54 P.M.; the present segment was 65 μ long at 6.15, and was 109 μ long at 7.6. In both cases, therefore, exactly the same time, i.e., fifty-one minutes, was occupied in growing the 44 u referred to, in the first case the temperature being 19° to 18.9° C., and in the second 19.25-19.5-19° C.

Then, again, the doubling period is the same as the second one of the original filament, viz., sixty-eight minutes, at nearly the same temperature.

Here the doubling period is becoming extended to more than double the previous ones, indicating the spoiling of the drop either

On examining these three curves, AB, CD, and EF,* we see that— 1. The general tendency of the weak gelatine-i.e., poor in food materials but stiff in consistency (Curve AB)-is to lengthen the doubling period.

2. That considerable irregularities occur at all the temperatures higher than about 24° C. or 25° C. These are no doubt due to the inequalities in the phase of growth, different ages of the culture, and so on, already discussed.

3. It looks as if the curve suddenly shot up at temperatures beyond 31-32° C., as shown by the curve for broth (CD).

4. It might, perhaps, be suggested that although the addition of gelatine delays the doubling period, it also delays the attainment of the maximum temperature, i.e., it protects the food-drop against exhaustion, until a higher temperature than that at which exhaustion sets in when broth alone is used. This may be the case, but I do not regard it as proved, and there are many difficulties with brothcultures at higher temperatures, as already said.

But (5) it is clear that these three curves, unsatisfactory though they are as yet in detail, show striking general resemblances to a curve of respiratory activity-the steady fall in the doubling period as we pass from lower temperatures, and the sudden rise again beyond 31° C. (seen in the broth curve) being very suggestive-and I believe that is what they really are, for reasons already discussed.

Having so far cleared the way for the understanding of these curves, we shall see that when attention is carefully given to all the points discussed, a much more regular curve is obtained, as the following series show.

* Curve AB summarises the averages for weak gelatine; CD, those for broth + 1 per cent. gelatine; EF, those for broth alone. The ordinates are minutes and the abscissæ temperatures.