THE CULTUEE OF FARM CHOPS. 



CHAPTER IX. 



HEAT AND COLD. THEIR INFLUENCE UPON MATTER 

 AND VEGETATION. 



Heat and cold are merely relative terms. Cold is a low 

 degree or absence of heat, just as darkness is the absence of 

 light, and has not in any sense, or in fact, any specific ex- 

 istence, as separate from heat. It is only quite recently 

 that the nature of heat has been understood. It was for- 

 merly supposed to be an element, a subtle fluid to which the 

 name Caloric was given ; and whose entrance into a body 

 produced warmth and whose loss produced cold. As some 

 bodies, such as marble, felt cold and others, as wool, felt 

 warm, it was believed that various substances contained less 

 or more of this fluid stored up in its interstices according to 

 their varying capacities. It was given, in fact, all the 

 properties of a gas with some others which were believed to 

 belong to it specifically. This ancient notion was exploded 

 when it was discovered that heat was simply the effect of 

 motion of the particles of a body, and that the intensity of 

 the motion determines the temperature. 



It is not the purpose here to discuss the various theories 

 which are held in regard to the nature of heat ; these may 

 beetudied in special works on the subject such as that of 

 Prof. Tyndall. It is most important for us to consider how 

 it affects those elementary and compound bodies which have 

 a close relation to the growth of plants, and its effects upon 

 germination and plant growth. It will be sufficient here 

 perhaps to repeat the words of Dr. Locke uttered a hun- 

 dred years ago in which the true idea of heat was enuncia- 

 ted. He said, "heat is a very brisk agitation of the insen- 

 sible parts of any object which produces in us that sensation 

 from which we call the object, hot; so that what in our 

 sensations is heat, in the object is nothing but motion." A 



