FOUNDATION FACTS AND PRINCIPLES 7 



cent, of phosphorus (the atomic weight of phosphorus being 31), 

 while the phosphorus pentachlorid contains less than 15 per cent 

 of phosphorus. 



Nitrogen and phosphorus are in some respects very much alike, 

 and in other respects they are very unlike. They are the two most 

 precious elements of plant food, and they deserve from the author 

 and from the reader all of the consideration they are to receive in 

 this book. 



The gas law. This law is that equal volumes of gases under like 

 conditions of temperature and pressure contain the same number 

 of molecules. In other words, in the gas form, every molecule 

 occupies, or controls, the same amount of space. Thus, the hydro- 

 gen molecule, with a weight of 2, occupies as much space as the 

 oxygen molecule, which weighs 32, or the molecule of carbon dioxid, 

 weighing 44, or of sulfur dioxid with a molecular weight of 64. 

 (The atomic weight of sulfur is 32.) 



If a 6-gallon bottle holds 2 grams of hydrogen (H 2 ), it will 

 hold 32 grams of oxygen (O 2 ), 28 grams of nitrogen (N 2 ), 16 grams 

 of methane (CH 4 ), 17 grams of ammonia gas (NHg), 44 grams of 

 carbon dioxid (CO^ , 64 grams of sulfur dioxid (SOg) ,and a gram- 

 molecule (the molecular weight in grams) of any other gas. This 

 law does not apply to liquids or solids, but only to gases. 



Chemical symbols. A symbol is used to represent one atom of 

 an element. H stands not only for the element hydrogen, but also 

 for one atom of hydrogen with a combining weight of i. Likewise 

 S stands for sulfur and for one atom of sulfur, and represents a 

 weight of .32. 



Chemical formulas. A formula is used to represent a molecule 

 and shows the kind and the number of atoms contained in the 

 molecule. The formula H 2 O represents one molecule of water, 

 containing two atoms of hydrogen, each having a combining 

 weight of i, and one atom of oxygen, with a weight of 16. Thus, 

 the molecular weight of water is 18. The formula Cag(PO 4 ) 2 

 (read: Ca three, PO four, twice) represents one molecule of 

 tricalcium phosphate, the valuable phosphorus compound contained 

 in bones and in natural phosphate rock. The metallic element 

 calcium (Ca) is also contained in limestone, which is calcium car- 

 bonate (CaCOs) , and in quicklime, or burned lime, which is calcium 



