432 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
Both temperatures are plotted in fig. 5, and the difference between them 
gives a measure of the intensity of the solar radiation at any time. 
Several errors and mishaps vitiate the middle part of this experiment, 
but it is, nevertheless, valuable; there is a very instructive difference 
between the effect of raising the temperature near noon, when the light 
is strong and the temperature limiting, and of again raising it late in the 
afternoon, when the light itself is hmiting. 
In the first two readings the bath-temperature is about 18° C., and the. leaf- 
temperature 4° to 2°°5 higher, giving, firstly, 0°0131 at the average temperature 
of 22°-2 C., and secondly 0:0109 at 20°77 C. The day is now bright, though 
not clear of clouds. The light is much in excess of the above values, and 
they are taken as maximal for their respective temperatures (see fig. 2). They 
fall into one curve with the value of 00090 at 18° C. obtained in Experiment X. 
After these two readings hotter water was circulated through the bath to 
give a temperature of about 27° C. The sun was irregular, and its outbursts 
show clearly on the temperature records, the leaf responding more acutely 
than the bath.. The value 0:0275 was recorded at 31°2 C. This large 
number is not, however, maximal, though the light is bright enough, and 
it was undoubtedly limited by a quite unusual factor—the CO2 supply. Not 
anticipating such vigorous assimilation, we had only provided 00191 gramme 
COz per half-hour, and the table shows that the leat had absorbed as much as 
00180. There is thus quite an inadequate margin of supply, and presently the 
CO, was increased. The fourth reading falls into quite dull illumination, as 
the closeness of the two temperature curves shows, and is no doubt limited by 
the light, giving only 0°0184 at 30° C. (cf fig. 5). 
After these readings the bath was cooled again. The fifth reading must be 
due to an undetected error of some sort, for it appears to be much above the 
maximum for the recorded temperature, while the sixth is normal again, and 
just about the same as the first, in temperature and assimilation, though 
light is much brighter and temperature-difference is large. Then a second 
time the bath is heated up and the assimilation rises to 0°0222 at 29° C. 
The next reading is spoilt by a drop in the temperature due to accidental 
extinction of the gas of the water-heater. 
Once more the bath is cooled down, and as the sky is thickly clouded the 
temperature-difference is now very small. The assimilation, 0:0084, is a 
little below the maximum for 17°7, and the light should be the limiting 
factor. This is made evident by heating up again for the last time, when in 
spite of a return to the temperatures about 29° C., there is, in contrast, no 
rise of assimilation, but a further fall with the decreasing light, which also 
reduces considerably the temperature-difference. 
