AGRICULTURAL CHEMISTRY. 16 



Also, calcium, an element, will combine with the ele- 

 ment, oxygen, precisely 20 lbs. of the first to 8 lbs. 

 of the last, and form 28 lbs. of quick-lime. Now, if 

 we take these two compounds, water and quick-lime, 

 9 lbs. of the former will combine with 28 lbs. of the 

 latter, and form 37 lbs. of slacked lime. It is true, you 

 might put more than 9 lbs. of water to 28 lbs. of lime, 

 but the excess would soon evaporate, leaving precisely 

 9 lbs. combined with the lime in the form of a dry, 

 white powder, (water-slacked lime). If you were to 

 put less than 9 lbs. of water to 28 of lime, then only 

 a part of the lime would be slacked ; and in order to 

 slack the whole, you would have to continue putting 

 on water till you had reached the 9 lbs., when the 

 whole would be reduced to a dry, white powder. It 

 is so with all chemical combinations ; they are always 

 in definite, fixed and unalterable proportions. In this 

 respect they differ from mere mixtures, which may be 

 in any proportions. 



ELEMENTS. 



18. There are in nature 15 simple substances, call- 

 ed elements, whicn make up more than 99 hundredths 

 of tall known matter. Other substances exist in small 

 quantity, but these are all that need be noticed in an 

 introduction to agricultural chemistry. They consti- 

 tute essentially all the objects with which we are con- 

 versant. If we analyze a stone, a handful of earth, 

 a plant, a flower, a bone, a drop of water, a piece of 

 flesh, almost anything we can think of, it is found to 

 consist of one, two^ three or more of these ; seldom of 



