New York Agricultural Experiment Station. 41 



Phosphoric acid consists oi phosphorus and oxygen and hydrogen^ 

 in place of hydrogen put one of the metals, as calcium^ and we have 

 a compound containing 



Phosphorus and ovyygen and calcium (in place of hydrogen), which 

 is the calcium salt of p>hosphoric acid and is called calcium p)hos- 

 phate or phosphate of lime. 



Similarly, if a metal, as magnesium, is put in the place of the 

 hydrogen of sulphuric acid, we have the magnesium salt of sul- 

 phuric acid or tnagnesium sidphate, familiar to us as Epsom salt. 

 If in hydrochloric (muriatic) acid, we put some metal as sodium in 

 place of the hydrogen, we have a compound consisting of sodium 

 and chlorine, which is the sodium salt of hydrochloric acid and is 

 called sodium chloride, sometimes muriate of soda, familiar to us as 

 common salt. 



The word "salt" as used in chemistry applies to a great number 

 of compounds, and many of the substances we have to deal with in 

 speaking of fertilizers are chemical salts i that is, substances formed 

 hy pidiing some metal in place of the hydrogen of some acid. 



Carbon. — («) Importance. — The element carbon may be called 

 the central element of all animal and vegetable substances ; for there 

 is not a living thing, from the smallest cell to the giant tree, which 

 does not contain carbon as a necessary constituent. That all vege- 

 table and animal substances contain carbon can easily be sliown by 

 simply heating them sufficiently, and thus causing them to hlacken 

 or char. When, for example, wood is heated, the different elements 

 of which it is composed are driven off in one form or another, but 

 the carbon is the last to go, and remains behind as a black substance 

 or charcoal, unless heated higher, when it disappears or burns up. 



(J) Occurrence. — Carbon usually occurs in nature united in com- 

 pounds with other elements. Thus, most products of plant life con- 

 tain carbon combined with the elements hydrogen and oxygen ; such 

 are starch, sugar, and cellulose or woody fibre. Carbon combined 

 with oxygen occurs in the air in the form of carbon dioxide, com- 

 monly called carbonic acid gas. Carbon, when combined with 

 ogygen and some element such as calcium, occurs in the form of 

 carbonates ; for example, marble, limestone, and chalk are chemically 

 known as calcium carbonate or carbonate of lime. 



Carbon by itself or in the free condition, that is, not united with 

 any other elements, is familiar to us in several different forms; the 

 most common of these are (1) diamonds ; (2) graphite, which is used 



