62 



AORlCULTUaAL CHEMISTRY. 



that the number of the revolutions could 

 be regulated ; the beans were supplied 

 with moisture, and were placed under cir- 

 cumstances favorable to germination. The 

 greatest velocity of motion given to the 

 wheel was such that it performed 2.50 

 revolutions in a minute. It was found 

 that in all cases the beans grew, and 

 that the direction of the roots and stems 

 was influenced by the motion of the wheel. 

 When the centrifugal force was made 

 superior to the force of gravitation, which 

 was supposed to be done when the vertical 

 wheel performed one hundred and fifty 

 revolutions in a minute, all the radicles, 

 in whatever way they were protruded 

 from the position of the seeds, turned 

 their points outwards fiom the circum- 

 ference of the wheel, and in their subse- 

 quent growth receded nearly at right 

 angles from its axis ; the germens, on the 

 contrary, took the opposite direction, and 

 in a few days their points all met in the 

 centre of the wheel. 



When the centrifugal force was made 

 merely to modify the force of gravitation 

 in the horizontal wheel when the greatest 

 velocity of revolution was given, the 

 radicles pointed downwards about ten 

 degrees below, and the germens as many 

 degrees above the horizontal line of the 

 wheels motion; and the deviation from 

 the perpendicular was less in proportion 

 as the motion was less rapid. 



These facts afford a rational solution of 

 this curious problem, respecting which 

 different philosophers have given such 

 different opinions ; some referring it to 

 the nature of the sap, as De la Hire, 

 others, as Darwin, to the living powers of 

 the plant, and the stimulus of air upon the 

 leaves, and of moisture upon the roots. 

 The effect is now shown to be connected 

 with mechanical causes; and there seems 

 no other power in nature to which it can 

 with propriety be referred but gravity, 

 which acts universally, and which must 

 tend to dispose the parts to take a uniform 

 direction. 



If plants in general owe their perpen- 

 dicular direction to gravity, it is evident 

 that the number of plants upon a given 

 part of the earth's circumference, cannot 

 be increased by making the surface irregu- 

 lar, as some persons have supposed. IS'or 

 can more stalks rise on a hill than on a 

 /spot equal to its base ; for the slight effect 



of the attraction of the hill, would be only 

 to make the plants deviate a very little 

 from the perpendicular. Where horizon- 

 tal layers are pushed forth, as in certain 

 grasses, particularly such as the fiorin, 

 lately brought into notice by Dr. Richard- 

 son ; more food may, however, be pro- 

 duced upon an irregular surface ; but the 

 principle seems to apply strictly to corn 

 crops. 



The direction of the radicles and ger- 

 mens is such that both are supplied with 

 food, and acted upon by those external 

 agents which are necessary for their 

 development and growth. The roots 

 come in contact with the fluids in the 

 ground ; the leaves are exposed to light 

 and air : and the same grand law which 

 preserves the planets in tlieir orbits, is thus 

 essential to the functions of vegetable life. 

 When two pieces of polished glass are' 

 pressed together they adhere to each other, 

 and it requires some force to separate 

 them. This is said to depend upon the 

 attraction of cohesion. The same attrac- 

 tion gives the globular form to drops of 

 water, and enables fluids to rise in capill- 

 ary tubes; and hence it is sometimes 

 cdiWcd capillary attraction. This attrac- 

 tion like gravitation seems common 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 fbrms of aggregation of the 

 parts of plants, and seems to be a principal 

 cause of tlie absorption of fluids by their 

 roots. 



If some pure magnesia, the calcined 

 magnesia of druggists be thrown into 

 distilled vinegar it gradually diss ves. — 

 This is said to be owing to cheynical 

 attraction.) the power by which different 

 species of matter tend to unite into one 

 compound. Various kinds of matter unite 

 with different degrees of 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 vine- 

 gar have been destroyed by the magnesia 

 the vinegar will be set free, and the sul- 

 phuric acid will take its place. This 

 chemical attraction is likewise called 

 chemical affinity. It is active in most of 

 the phenomena of vegetation. The sap 

 consists of a number of ingredients, dis- 



