66 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
Section (4).—(a) Huperiments on Leaves Exposed to Artificial Illumination in 
a Current of Ordinary Arr. 
These experiments were made during the winter months of 1900-1901 
with cut leaves of greenhouse-plants illuminated on the upper sides with 
light from a large No. 4 Welsbach burner. 
They were instituted with the idea of working out the influence of varying 
external conditions on the assimilatory process under constant illumination, 
but failed in this object owing to the comparatively feeble photosynthetic 
power of the source of light used, and the abnormal respiration which occurs 
at this time of the year in greenhouse-plants. These two causes resulted in a 
complete masking of the assimilatory process as measured in the ordinary 
way, the air which had been in contact with the leaf always gaining in carbon 
dioxide owing to respiration being in excess of assimilation. 
More satisfactory results would, no doubt, have been obtained by this 
method in the summer, but this line of research was abandoned when it was 
found that definite variations in the intensity of sunlight or diffused daylight 
could be conveniently produced by the revolving radial sectors already 
described, thus admitting of comparisons being made in simultaneous 
experiments lasting for some hours, although the actual intensity of the 
unobstructed radiation might vary considerably during that time. 
Nevertheless some of these experiments with artificial illumination are 
not devoid of interest, and we have, therefore, briefly alluded to a few 
of them. 
The actual heating effect* of a large Welsbach burner placed within a foot 
or two of a leaf enclosed in its glazed case is considerable, and had to be 
neutralised as far as possible by passing the light through a water-cell 2 or 
3 inches in thickness. Where comparisons were made with the respiration 
effect in darkness, the respiratory chamber was as far as possible brought 
to the same temperature as the one in which the assimilatory effect was 
investigated. 
The leaf was enclosed as usual in its case through which a measured stream 
of air was aspirated, of which the carbon dioxide-content was determined at 
ingress and egress, and the case was suitably arranged so that no light could 
reach the leaf except that coming from the gas burner. 
The temperature of the air in the case was given by a thermometer inserted 
* The total radiation from the Welsbach burner employed, at a distance of 1 foot and with 
only a thin glass screen, was found by the Callendar’s radiometer to be 0°089 cal. per sq. em. per 
minute. When the water-cell was interposed this fell to 0°031 cal. per sq. cm. per minute. Ata 
distance of 2 feet these values were, of course, reduced to one quarter. 
