1S38] 



FARMERS' REGISTER 



685 



Fi?. 1 



attraction gives the globular form to drops of wa- 

 ter, and enables fluids to rise in capillary tubes; 

 and hence it is sometimes called capillary attrac- 

 tion. This attraction, like gravitation, seems com- 

 mon to all matter, and may be a modification of 

 the same general force; like gravitation, it is of 

 great importance in vegetation. It preserves the 

 forms of aggregation of the parts of plants, and it 

 seems to be a principal cause of the absorption of 

 fluids by their roots. 



If some pure magnesia, the calcined magnesia 

 of druggists, be thrown into distilled vinegar, it 

 gradually dissolves. This is said to be owing to 

 chemical attraction, the power by which different 

 species of matter tend to unite into one compound. 

 Various kinds of matter unite with different de- 

 grees ot force: thus sulphuric acid and magnesia 

 unite with more readiness than distilled vinegar 

 and magnesia; and if sulphuric acid be poured 

 into a mixture of vinegar and magnesia, in which 

 the acid properties of the vinegar have been de- 

 stroyed by the magnesia, the vinegar will be set 

 free, and the sulphuric acid will take its place. 

 This chemical attraction is likewise called chemi- 

 cal affinity. It is active in most of the phenome- 

 na of vegetation. The sap consists of a number 

 ' of ingredients, dissolved in water by chemical at- 

 traction; and it appears to be in consequence of 

 the operation of this power, that certain principles 

 derived from the sap are united to the vegetable 

 organs. By the laws of chemical attraction, dif- 

 ferent products of vegetation are changed, and as- 

 sume new forms: the food of plants is prepared in 

 Vol. VI. -74 



the soil; vegetable and animal remains are changed 

 by the action of air and water, and made fluid or 

 aorilbrm; rocks are broken down and converted 

 into soils; and soils are more finely divided and 

 fitted as receptacles for the roots of plants. 



The different powers of attraction tend to pre- 

 serve the arrangements of matter, or to unite them 

 in new forms. If there were no opposing powers 

 there would soon be a state of perfect quiescence 

 in nature, a kind of eternal sleep in the physical 

 world. G ravitation is continually counteracted by 

 mechanical powers, by projectile motion, or the 

 centrifugal force; and their joint agencies occa- 

 sion the motion of the heavenly bodies. Cohe- 

 sion and chemical attraction are opposed by the 

 repulsive energy of heat, and the harmonious cycle 

 of terrestrial changes is produced by their mutual 

 operations. 



Heat is capable of being communicated from 

 one body to other bodies ; and its common eflfect 

 is to expand them, to enlarge them in all their di- 

 mensions. 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 healed in a globe of glass 

 having a long slender neck, it rises in the neck ; 

 and if heat be applied to air confined in such a 

 vessel inverted above water, it makes its escape 

 from the vessel and passes through the water. 

 Thermometers are instruments for measuring de- 

 grees of heat by the expansion of fluids in nar- 

 row tubes. Mercury is generally used, of which 

 100,000 parts at the freezing point of water become 

 101,835 parts at the boiling point, and on Fahren- 

 heit's scale these parts are divided into 180 de- 

 grees. Solids, by a certain increase of heat, be- 

 come fluids, and fluids gases; or elastic fluids. 

 Thus ice is converted by heat into water, and by 

 still more heat it becomes steam; and heat disap- 

 pears, or, as it is called, is rendered latent, during 

 the conversion of solids into fluids, or fluids into 

 gases, and re-appears, or becomes sensible, when 

 gases become fluids, or fluids solids ; hence cold 

 is produced during evaporation, and heat during 

 the condensation of steam. 



There are a few exceptions to the law of ex- 

 pansion of bodies by heat, which seem to depend 

 either upon some change in their chemical con- 

 stitution, or on their becoming crystallised. Clay 

 contracts by heat, which seems to be owing to its 

 giving off' water. Cast iron and antimony, when 

 melted, crystallise, 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 at about 4P or 42'^, the freezing point 

 being 32° ; and this circumstance is of consider- 

 able importance in the general economy of nature. 

 The influence of the changes of seasons and of 

 the position of the sun on the phenomena of ve- 

 getation demonstrates the effiscts 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 de- 

 rive no nourishment from it. The activity of che- 

 mical changes likewise 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 shown by placing in each 

 of tv/o wine glasses a similar hollow stalk of 

 grass, so bent as to discharge any fluid in the 

 glasses slowly by capillary attraction : if hot water 



