THE CHEMICAL RADIATIONS. 



25 



to produce pbotogriiphic effects which in New York require only a 

 minute; and the further statement of travelers, engaged in copying 

 the antiquities of Yucatan, that they frequently have been obliged to 

 abandon the use of the camera, and take to their sketch-books, have 

 led to some investigations, similar to those at Kew, for determining 

 the intensity of the chemically-active rays in the tropics. Prof. 

 Thorpe experimented at Para, situated nearly under the equator, in the 

 northern province of the Brazils, and lying on a branch of the Ama- 

 zon. Of the results. Prof, lloscoe remarks: "Owing to the rainy sear 

 son having commenced when the experiments were made, the changes 

 in the chemical intensity, as observed from hour to hour, and even from 

 minute to minute, are very sudden and remarkable; this is well shown 

 by the zigzag lines of Figs. 5 and 6 ; and these, compared with the 

 dotted lines below, indicating the corresponding action on the same 



Figs. 5 and 6 Variation of Chemical Eays in the Tropics. 



day at Kew, show the enormous variation in chemical intensity which 

 occurs under a tropical sun in the rainy season. Regularly every 

 afternoon, and frequently at other hours of the day, enormous thun- 

 der-clouds obscure the sky, and, discharging their contents in the 

 form of deluging rain, reduce the chemical action nearly to zero. The 

 storm quickly passes over, and the chemical intensity rapidly rises to 

 Its normal value. By comparing the curves for Para and Kew on the 

 same days, we obtain some idea of the energy of chemical action at 

 the tropics, and it is at once evident that the alleged failure of the 

 photographer cannot at any rate be ascribed to a dimimition in the 

 sun's chemical intensity, which, in the month of April, 1866, was 

 nearly seven times as great at Para as at Kew." 



