AGRICULTURAL CHEMISTRY. 



63 



solved in water by chemical attraction ; 

 and it appears to be in consequence of the 

 operation of this power, that certain prin- 

 ciples derived from sap are united to the 

 vegetable organs. By the laws of chemi- 

 cal attraction, different products of vege- 

 tation are changed, and assume new forms; 

 the food of plants is prepare;! in the soil ; 

 vegetable and animal remains are changed 

 by the action of air and water and made 

 fluid or aeriform ; rocks are broken down 

 and converted into soils ; and soils are 

 more finely divided and fitted as recepta- 

 cles for the roots of plants. 



The ditferent powers of attraction tend 

 to preserve the arrangements of matter, 

 or to unite them in new forms If tiiero 

 were no opposing powers, there would 

 soon be a state of perfect quiescence in 

 nature, a kind of eternal sleep in the 

 physical world. Gravitation is continually 

 counteracted by mechanicnl agencies, by 

 projectile motion on the centrifugal force; 

 and their joint agencies occasion the mo- 

 tion of the heavenly bodies. Cohesion 

 and chemical attraction are opposed by 

 the repulsive energy of heat, and, the 

 harmonious cycle of terrestrial changes 

 is produced by their mutual o])erations. 

 - Heat is capable of being communicated 

 from one body to other bodies ; and its 

 common effect is to expand them, to 

 enlarge them in all their dimensions. This 

 is easily exemplified. A solid cylinder 

 of metal after being heated will not pass 

 through a ring barely sufficient to receive 

 it when cold. When water is heated in a 

 globe of glass having a long slender neck, 

 it rises in the neck and if heat be applied 

 in air confined in such a vessel inverted 

 above water, it makes its escai)e from the 

 vessel and passes through the water. Ther- 

 mometors are instruments for measuring 

 degrees of heat by the expansion of fluids 

 in narrow tubes. Mercury is generally 

 used, of which 100,000 parts at the freez- 

 ing point of water become 10!, 835 parts 

 at the boiling point, and on Fahrenheit's 

 scale these parts are divided into hundred 

 and eighty degrees. 



Solids, by a certain increase of heat, 

 become fluids, and fluids gasses, or elastic 

 fluids. Thus ice is converted by heat into 

 water, and by still more heat it becomes 

 steam: and heat disaj)pears, or, as it is 

 called, is rendered latent daring the con- 



version of solids into fluids, or fluids into 

 gasses, and reappears or becomes sensible 

 when gasses become fluids, or fluids solids: 

 hence cold is produced during evaporation, 

 and heat by the condensation of steam. 



There are a few exceptions to the law of 

 expansion of bodies by heat, which seem 

 to depend either upon some change in 

 their chemical constitution, or on their 

 becoming crystallized. Clay contracts by 

 heat, which seems to be owing to its giving 

 ofl" water. Cast iron and antimony, when 

 melted, crystallize in cooling and expand. 

 Ice is much lighter than water. Water 

 expands a little even before it freezes, and 

 it is of the" greatest density atabout 41°or 

 42°, the freezing point being 32° ; and 

 this circumstance is of considerable im- 

 portance in the general economy of nature. 

 The influence of the changes of seasons 

 and of the position of the sun on the 

 phenomena of vegetation, demonstrates 

 the effects of heat on the functions of 

 plants. The matter absorbed from the 

 soil must be in a fluid state to pass into 

 their roots, and when the surface is frozen 

 they can derive no nourishment from it. 



The activity of chemical changes like- 

 wise is increased by a certain increase of 

 temperature, and even the rapidity of the 

 ascent of fluids by capillary attraction. 



This last fact is easily shovvn by placing 

 in each of two wine glasses a similar 

 hollow stalk of grass, so bent as to dis- 

 charge any fluid in the glasses slowly by 

 capillary attraction ; if hot water be in 

 one glass, and cold in the other, the hot 

 water will be discharged much more 

 rapidly than the cold water. 



The fermentation and decomposition of 

 animal and vegetable substances require a 

 certain degree of heat, which is conse- 

 quently necessary for the preparation of 

 the food of plants ; and as evaporation is 

 more rapid in proportion as the tempera- 

 ture is higher, the superfluous parts of the 

 sap are the 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 

 particles of other matter. By others it is 

 considered as a motion or vibration of the 

 particles of matter, which is supposed tq 



