3IO 



Transactions of thk Canadian Institutk. 



I Vol. VII. 



The leaves were in the open air without protection against too rapid 

 transpiration, and in consequence there was a greater amount of 

 crystalline residue than might otherwise have been expected. Those 

 substances recorded as moist were so twenty-four hours after the drop 

 was placed upon the leaf, and they remained so for a longer time, though 

 no further record was made. The fact that a darker ring occurred was 

 noted particularly because of the peculiar formation of the crystalline 

 deposit formed as the drop evaporates. 



Boussingault and Schlossing 

 ^ . noticed this ring of cr)'stals, and 



Boussingault noticed also that 

 in some cases of more dilute 

 solutions, a darker green ring 

 was produced on the leaf by the 

 drop of solution. No attempt 

 was made by Boussingault or 

 any one else to explain this, so 

 it was thought well to make the 

 point clear, although it belongs 

 properly to physics. 



When a solution is applied 

 to a leaf surface in the form of a 

 drop, the salt upon rapid eva- 

 poration of the water, forms 

 itself in a visible ring, which re- 

 sults in an unusually strong 

 action upon the leaf tissue im- 

 mediately under this ring of 

 crystals. This ring formation is 

 not confined to leaves, but is 

 true of crystalline substances in 

 general. If a drop of a salt 

 solution be placed upon a clean 

 glass slide and allowed to 

 evaporate, it is found that there 

 of the original drop, while inside 

 evenly spread over the surface. 



FIG. lo. 



XY is a section of a glass slide upon which is 

 a drop of solution BCD, which t,^kes the form 

 roughly of a flattened spheroid touching the glass in 

 the form of a circle with two points on the circum- 

 ference, as B and D. As the drop evaporates it 

 assumes roughly the form BMD and BND, the 

 points B and D remaining, and the circle of crystals 

 forms roughly as shown in S' S", C C". The curve 

 BFA being sharper than the curve CE, there will be 

 more rapid evaporation and consequently crystals 

 will form first at B .ind D, where they remain forming 

 the ring. Owing to cohesion and tendency to 

 crystallize, the first crystals formed attract solution 

 until almost all evaporates. Z, ground plan of drop. 



is a ring of salt at, or near, the edge 

 the ring there are very minute crystals 



Let XY (see Fig. lo) represent a glass slide, and BCD a section of a 

 drop of solution. Because of the internal cohesion of the particles of 

 the liquid, these tend to form a sphere, but this tendency being in part 



