Polytechnic Association. 591 



Dr. Yan der Weyde — If you try to ignite gunpowder by means of 

 an electric spark from a machine producing electricity of tension, it 

 will not burn. But take a wet thread, and pass the current through 

 it so as to make the spark move slowly, and it will ignite. A large 

 Euinkorff coil will not ignite a match. The electricity flies around 

 it. But dip the match in water, and it will ignite at onc$. 



Prof. Phin — Place the substance between two wires, and you can 

 ignite either the powder or the match. 



YI. Exhalation of Plants. 



The functions of the leaf are such that, during its exposure to sun- 

 light, it gives off exhalations both of gas and vapor. It decomposes 

 carbonic acid gas, absorbing the carbon and setting free the other 

 component, oxj'gen gas ; at the same time it concentrates the sap of 

 the plant by carrying oif its surplus water through the pores of the 

 leaf in the form of vapor. During early vegetation, the quantity of 

 water exhaled by the plant is very great. Botanists have carefully 

 measured the extent of this exhalation in certain plants. Several 

 experiments of Hales and others may here be noted. A sunflower, 

 three and a half feet high, presenting a surface of thirty-nine square 

 feet, and exposed to the air and light, was found to perspire at the 

 rate of from twenty to thirty ounces avoirdupois during every 

 twelve hours, or about seventeen times more than the amount per- 

 spired by an ordinary-sized man during the same time. An apple 

 tree with twelve square feet of foliage was found to perspire nine 

 ounces of water per day, and a vine of about the same surface from 

 five to six ounces. Recent experiments by Dr. McNab with the 

 laurel cherry prove that its leaves contain about sixty-three per cent 

 of water. Sunlight was found to be more efficient than chloride of 

 lime or sulphuric acid in extracting water from the leaf. In light 

 of any kind the under-surface of the leaf was found to perspire more 

 water than the upper surface. At night the process is arrested, and 

 even in the shade only two per cent of the, water in the leaf passed 

 off per hour into a dry atmosphere, while in a saturated atmosphere 

 exhalation ceased. It seems obvious that this function of the leaf 

 must have great effect in modifying climate. Experiments in India 

 and Africa in planting extensive forests in territory deficient in 

 moisture have shown that within a few years the number of rainy 

 days during the year have increased at least fourfold. 



Dr. Yan der Weyde — I think the attraction of trees for moisture 



