Polytechnic Association. 593 



by trees, which entirely dried up when the trees were cut down. On 

 planting new trees there, on the hills, the water returned, and increased 

 as the trees increased. 



Dr. Tan der "Weyde — The number of rainy days in Egypt has been 

 increased by the planting of trees. And the French have made artesian 

 wells in the deserts of Algeria and created oases there. I do not 

 think we need look to electricity for the cause of the rain. The cool- 

 ness of the air, in forests, seems to me a much stronger source of the 

 precipitation of water than any mysterious electrical tricks. "When 

 warm atmosphere fully charged with moisture comes in the neighbor- 

 hood of the cool forests, there is, of course, a tendency to precipitation. 



Prof. Phin — The difficulty with regard to the theory of coldness is 

 simply this, that in winter plants are invariably warmer than the 

 surrounding atmosphere, and that with the exception of hot days 

 their temperature is always higher. When we have rain, the tem- 

 perature of forests is always higher. On warm days, when the 

 atmosphere is dry and carrying off the moisture, their temperature is 

 lower ; but then we have no rain. On cool days, when we have rain, 

 their temperature is higher. 



Dr. Yan der Weyde — They are cooler on warm days in consequence 

 of the evaporation ; for evaporation is a cooling process. And when 

 they are cooled, they are in a condition to condense the moisture from 

 a current of warm and saturated air. 



YII. Delicacy of the Spectrum Analytical Method. 

 Prof. Roscoe, in a recent lecture " On Spectrum Analysis and its 

 Application to the Bessemer Process of Manufacturing Steel," pre- 

 sented the following table, showing that, in the spectrum analysis 

 method, the delicacy of reaction is something almost beyond 

 belief : 



1. Soda.— l-3,000,000th of a milligramme or l-lS0,000,000th of a 

 grain of soda can be easily detected. Soda is always present in the 

 air. All bodies exposed to the air show the yellow soda line. If a 

 book be dusted near the flame, the soda reaction will be seen. 



2. Lithia.— l-100,000th of a milligramme of lithia or l-6,000,000th 

 of a grain can be easily detected. Lithium was only known to occur 

 in four minerals. It is now found, by spectrum analysis, to be one of 

 the most widely distributed elements. It exists in almost all rocks, in 

 sea and river water, in the ashes of most plants, in milk, in human 

 blood, and muscular tissue. 



3. Strontia.— 6-100,000 of a milligramme or 1-1 ,000,000th of a 



[Inst.] 38 



