LAW OF MULTIPLE PROPORTIONS. 35 



carbonic acid are constant and invariable under whatever circumstances 

 they may be formed, and the elements of which they consist, when they 

 combine together in the same proportions, are never known to form any 

 other compounds but water and carbonic acid. 



This law, however, thoi2gh generally, is not universally true. Many 

 substances are known which contain the same elements united together 

 in the same proportions, and which, nevertheless, possess very different 

 properties. Oil of turpentine and oil of lemons are in this condition. 

 They both consist of the same elements, carbon and hydrogen, united 

 together in the same proportions, and j^et their sensible properties as well 

 as their chemical relations* are very dissimilar. 



Cane sugar, starch, and gum, all of them abundant products of the 

 vegetable kingdom, consist also of the same elements, carbon, hydro- 

 gen, and oxygen, united together in the same proportions, and may even 

 be represented by the same form^da (C^a Hio Oio)it ^nd yet these 

 substances are as unlike to each other in their properties, as many 

 bodies are of which the chemical composition is very different. To 

 compounds thus differing in their properties, and yet containing the 

 same elements, in the same proportions, chemists hajjp given the name 

 of Isomeric bodies. I shall have occasion to make you more familiar 

 with some of them hereafter. 



3°. Another important law by which chemical combinations are 

 regulated, is known by the name of the law of multiple proportions. 

 Some substances are observed to be capable of uniting together in more 

 than one proportion. Thus carbon unites with oxygen in several pro- 

 portions, forming carbonic oxide, carbonic acid, oxalic acid, &c. Now 

 when such is the case, it is found that the quantity (the weight) of each 

 substance which enters into the several compounds, if not actually re- 

 presented by the equivalent number or atomic weight, is represented by 

 some simple multiple of that number. Thus two equivalents of carbon 

 unite with 2, 3, or 4 equivalents of oxygen, to form carbonic oxide, 

 oxalic acid, and carbonic acid respectively, — while one of nitrogen unites 

 with 1, 2, 3, 4, or 5 of oxygen to form a series of compounds, of which 

 the last (N O5), nitric acid, is the only one I shall have frequent occa- 

 sion to speak of in the present lectures. 



This law of multiple proportions, though of great importance ia 

 chemical theory, I do not further illustrate, as we shall have very little 

 occasion to refer to it in the discussion of the several topics which will 

 hereafter come before us. 



Having thus briefly explained the nature and laws of chemical coin 

 bination, I proceed to make you acquainted with those chemical com- 

 pounds of the organic elements which are known or are supposed to 

 minister to the growth of plants. 



The number of compounds which the four organic elements form 

 with each other is almost endless ; but of this number a very few only 



' By the chemical relations of a substance are meant tae eflfects which are producei 

 upon it by contact with other chemical substances. 



t This formula means that starch, gum, and sugar, consist of 12 equivalents of carbon 

 united to 10 of hydrogen and 10 of oxygen. 



