[ Sfl ] 



proportion as the temperature is higher, the superflu- 

 ous parts of the sap are most readily carried off at the 

 time its ascent is quickest. 



Two opinions are current respecting the nature 

 of heat. By some philosophers it is .conceived to be 

 a peculiar subtile fluid, of which the particles repel 

 each other, but have a strong attraction for the parti- 

 cles of other matter. By others it is considered as 

 a motion or vibration of the particles of matter, 

 which is supposed to differ in velocity in different 

 cases, and thus to produce the different degrees of 

 temperature. Whatever decision be ultimately made 

 respecting these opinions, it is certain that there is 

 matter moving in the space between us and the hea- 

 venly bodies capable of communicating heat ; the mo- 

 tions of which are rectilineal : thus the solar rays 

 produce heat in acting on the surface of the earth. 

 The beautiful experiments of Dr. Herschel have 

 shewn that there are rays transmitted from the sun 

 which do not illuminate ; and which yet produce 

 more heat than the visible rays ; and Mr. Ritter and 

 Dr. Woliaston have shewn that there are other invisi* 

 ble rays .distinguished by their chemical effects. 



The different influence of the different solar rays 

 on vegetation have not yet been studied ; but it is cer- 

 tain that the rays exercise an influence independent of 

 the heat they produce. Thus plants kept in the dark 

 in a hot-house grow luxuriantly, but they never gain 

 their natural colours ; their leaves are white or pale, 

 and their juices watery and peculiarly saccharine. 



