THE VOLATILE PART OF PLANTS. 33 



Multiple Proportions. When two or more bodies 

 nnite in several proportions, their quantities, when not 

 expressed by the atomic weights, are twice, thrice, four, 

 or more times, these weights ; they are multiples of the 

 atomic weights by some simple number. Thus, carbon 

 and oxygen form two commonly occurring compounds, 

 viz., carbon monoxide, consisting of one atom of each in- 

 gredient, and carbon dioxide, which contains to one atom, 

 or 12 parts by weight, of carbon, two atoms, or 32 parts 

 by weight, of oxygen. 



Molecules* contain and consist of chemically-united 

 atoms, and are the smallest particles of matter that can 

 have an individual or physical existence. While the 

 atoms compose and give character to the molecules, the 

 molecules alone are sensibly known to us, and they give 

 character to matter as we find it in masses, either solid, 

 liquid or gaseous. In solids the molecules more or less 

 firmly cohere together ; in liquids they have but little 

 cohesion, and in gases they are far apart and tend to sepa- 

 rate from each other. The so-called "elements" are, in 

 fact, mostly compounds whose molecules consist of two 

 or more like atoms, while all other chemical substances 

 are compounds whose molecules are made up of two or 

 more unlike atoms. 



Molecular Weights of Compounds. The mole- 

 cular weight of a compound is the sum of the weights of 

 the atoms that compose it. For example, water being 

 composed of 1 atom, or 16 parts by weight, of oxygen, 

 and 2 atoms, or 2 parts by weight, of hydrogen, has the 

 molecular weight of 18. f 



The following scheme illustrates the molecular compo- 

 sition of a somewhat complex compound, one of the car- 



* Latin diminutive, signifying a little mass. 



t We must refer to recent treatises on chemistry for fuller informa- 

 tion as to atoms and molecules and the methods of finding the atomic 

 and molecular weights. 



