36 



branch," "or a few scattered leaves" in August mentioned 

 by Emerson have been fully accounted for in the preceding 

 pages, and it is unnecessary to remark further that such 

 cases have nothing whatever to do with the agent that trans- 

 forms the color of a whole forest almost in a night ! 



Of course it is to be understood that it is not frost alone, 

 but the alternate action of frost and sun, the sudden changes 

 of temperature, which derange the normal functions of the 

 leaf, and induce the chemical changes which accompany the 

 change of tint. 



Gray's Manual states that Quercus rubra, L. Red Oak, 

 turns dark red after frost ! 



A hoar frost, as is well known, "is the moisture of the air 

 condensed at freezing temperature upon plants and other 

 objects near the surface of the earth." It is not limited to 

 a general temperature of 32° Fahrenheit or below, but " maj^ 

 as a rule be expected when the thermometer indicates S'^ to 

 10" above the freezing point"! A. G. McAdie states it is 

 liable to occur at an air temperature of from 40° to 45° Fah- 

 renheit when the other conditions are favorable, as a clear 

 and still night and much moisture in the air. 



The cause of the frost is the radiation of heat which re- 

 duces the temperature of the surface of the earth and vege- 

 tation below that of the free air. Rapid evaporation also 

 reduces the temperature, as is shown by the difference often 

 seen between the wet and dry bulb of thermometers, which, 

 at times, may be as much as 12°, or more, lower in the wet 

 one, which more nearly represents the actual temperature 

 of foliage wet with night dews. 



The minimum temperature at New Bedford, Mass., on 

 three different nights for September, 1900, was SG-^, 39°, 

 43°. Upon an examination of the records of the City En- 

 gineer's Department for the preceding nine years, it was 

 found that in each year there were at least three days when 

 the minimum temperature was not above 42°, and ranging 

 from 35° to 42°, except in 1891, when the minima were 44°, 



