ATO 



ATO 



ATMOSPHERE. The bulk ot air 

 which surrounds our globe, supposed 

 to reach forty-five miles above its sur- 

 face. It is the receptacle of every 

 volatile substance rising from the 

 earth ; but. in virtue of its peculiar 

 composition, vapours and gases dif- 

 fuse themselves throughout the mass 

 with great rapidity, so that the com- 

 position of the whole is maintained 

 nearly uniform at all times and places. 

 Its chemical composition is 79 parts 

 nitrogen, 20 8 oxygen, 4 to 6 parts in 

 ten thousand of carbonic acid, about 

 one part in sixty thousand of ammo- 

 nia, according to Liebig, besides mi- 

 nute quantities of various vapours, 

 microscopic seeds, and saline mat- 

 ters. Water, in the form of vapour, 

 is also an important constituent, fluc- 

 tuating in quantity with the tempera- 

 ture of the air, and increasing as the 

 warmth. In the development of plants 

 the air is as important as the earth, 

 indeed more so, since many vege- 

 tables can live suspended, without 

 contact with the earth, while none 

 can exist without a full supply of air. 

 The loosening of soils is in a great 

 measure beneficial from the intro- 

 duction of air. The various ingre- 

 dients enumerated have not an equal 

 importance in agriculture ; for the ni- 

 trogen is almost inactive ; the oxj'- 

 gen is the great agent of destruction 

 as regards plants, causing the decom- 

 position of all vegetable structures ; 

 the carbonic acid and ammonia are 

 the great sources of food, and al- 

 though they are present in minute 

 proportions, they are abundantly dis- 

 tributed for the purposes of vegeta- 

 tion. For the liistory of these gases, 

 see them severally. 



ATOM. In chemistry, the ultimate 

 particle of a body, which combines 

 with other atoms. Theoretically, 

 these are of a determinate magni- 

 tude in every case. The figure of 

 the atom is not worthy of consider- 

 ation, some supposing it spherical, 

 others elipsoidal. Atoms are simple 

 or elementary when they cannot be 

 separated by chemical forces, and 

 compound when they are liable to de- 

 composition Chemical compounds 



E 



consist of a definite number of atoms, 

 bound together by chemical force or 

 affinity ; but the value of this force is 

 different in different compounds. In 

 consequence, however, of the union 

 of atoms in invariable weights, deter- 

 mined by experiment, each chemical 

 body has attached to it a distinct pro- 

 portional weight, termed its atomic 

 weight, equivalent, or combining num- 

 j ber. The study of these is the es- 

 sential of all chemical inquiries : it is 

 this remarkable adherence to a pre- 

 cise weight in all cases of combina- 

 tion which gives exactness to our in- 

 vestigations, and forms the difference 

 i between a mere mixture and chera- 

 j ical union. The following are the 

 1 atomic weights or proportionals of 

 the various elementary bodies inter- 

 ! esting to agriculture : 



Hydrogen (H.) . . . . 1- 



Oxygen (O.) 8- 



Nitrogen (N.) 14- 



Carbon (C.) 6- 



Sulphur (S.) 16- 



Phosphorus (P.) . . . .31- 



Chlorine (CI.) 35-5 



Silicon (Si.) 22- 



Potassium (K.) . . . .39* 



Sodium (Xa.) 23- 



Calcium (Ca.) 20-5 



Magnesium (Mg.) . . . 12-7 

 Aluminium (Al.) .... 13-7 



Iron (Fe.) 27- 



Manganese (Mn.) . . . 27-7 



These are on the basis that hydro- 

 gen is 1-, and may be understood by 

 the following case : Water is a com- 

 pound of one atom of hydrogen and 

 one atom of oxygen ; and, supposing 

 a given quantity weighs nine grains, 

 we know, by the laws of chemical 

 combinations, that it contains one 

 grain of hydrogen and eight grains 

 of oxygen ; or, if the weight of water 

 be other than nine grains, these con- 

 stituents are united in the rigorous 

 proportion of 1 to 8. 



Another scale is constructed on 

 the basis of oxygen as 100. In this 

 the equivalent numbers are altered, 

 but not their proportions. 



Chemical combinations are not, 

 however, always in the simple ratio 



