28 MR JOHN AITKEN ON DEW. 



others on the same plant were dry. These differences were noticed to be quite 

 irrespective either of their exposure to the sky, or to the probable humidity of 

 the air surrounding them. 



In illustration of this latter point, small clusters of dwarf French poppies 

 may be mentioned. Most of the plants were quite dry, whilst others growing 

 amongst them were dripping with moisture; and while some branches were 

 dry, others on the same plant were studded with drops, and the general 

 surface of the leaves in some cases wet. On examination of these plants 

 next day, it was observed that those that were wet at night were all plants 

 in vigorous growth, and the shoots that were dewed were those in which the 

 vegetation seemed most active. It was also observed that it was always the 

 same plants and branches that were dewed night after night during the 

 short time the observations were made. 



A closer examination of the leaves of broccoli plants showed better than 

 any others that the moisture collected on them was not deposited in the 

 manner we should expect if it had been deposited as an effect and according 

 to the laws of radiation ; nor was it deposited in accordance with the laws of 

 condensation; indeed, every appearance was at variance with these laws. 

 Examination showed that the moisture was collected in little drops placed at 

 short distances apart, along the very edge of the leaf, while the rest of the leaf 

 was often dry. Now, if the moisture had been condensed by cold produced by 

 radiation, then it would have been most abundant on the upper surface of the 

 leaf; but there would have been none on its windward edge. This is well 

 seen when we expose a small glass plate on a dewy night; the windward edge 

 is always dry, and the deposit is spread evenly over the rest of the plate up to 

 the opposite margin, because the temperature of the air when it first strikes 

 the plate is higher than the dew-point, and it has to travel over more or less 

 of the surface of the glass before it is cooled enough to deposit its moisture. 

 Again, if these drops on the edge of the leaf had been deposited according to 

 the laws of condensation, then the moisture would have been deposited on the 

 surface more in accordance with the distribution of temperature at the different 

 points; the moisture would therefore have been more equally distributed, and 

 not been in large isolated drops. 



On further examining these plants, I placed the lantern behind the blade, and 

 then observed that the position of the beautiful sparkling diamond-like drops 

 that fringed its edge had a definite relation to the structure of the leaf; they 

 were all placed at the points where the nearly colourless and semi-transparent 

 veins of the leaf came to the outer edge, at once suggesting that these veins 

 were the channels from which the drops had been expelled. 



These isolated drops on the edges of the leaves were therefore evidently 

 not dew, but an effect of the vitality of the plants. An examination of grass 



