ORGANIC CHEMISTRY. 297 



ties! One is sweet, crystalline, capable of fermenting, 

 neutral to litmus paper, being neither an acid, a base, nor 

 a salt ; the other is sour, liquid at ordinary temperatures, 

 and capable of combining with bases to form a large series 

 of salts. 



We have said these bodies differ in their molecular 

 weights ; we will explain why this is. You learned in 30 

 that the molecular weight of a body is equal to the sum of 

 its atomic weights; hence we calculate thus: 



Acetic acid. Grape Sugar. ' 



C - 12; C 3 = 2 C 6 = 72 



H = 1; II 4 = 4 H 18 - 12 



O = 16; O 3 = 32 O = 9G 



Molecular weight = CO Molecular weight = 180 



Substances which thus have the same chemical constitu- 

 tion, and yet are dissimilar in their qualities, are called iso- 

 meric substances, this term coming from two Greek words, 

 tsos, equal, and meros, part. 



Isomeric substances may even have the same molecular weight ; they 

 are then said to be metamerlc. 



Thus the molecular formula C 3 H 6 O 3 represents three different bodies 

 possessing different properties and different constitutions ; how this can be 

 is shown in the following formulas : 



C 3 H 6 O 2 may be arranged thus: C 3 H 5 O. HO which is Propionic Acid. 



" C 2 H 3 O.CH 3 " Methyl Acetate. 



" " CHO.C a H 3 O " Ethyl Formate. 



It is not necessary to know the nature of these bodies their names 

 show you that they are essentially distinct. One, you observe, is an acid, 

 the other two are compound ethers belonging to the class described in 

 423. 



416. Explanation of Isomerism. The explanation which 

 the atomic theory affords of this isomeric state is illustrated 

 by Stockhardt by the various grouping of white and black 

 squares which can be made on a chess-board, as seen in 



N 2 



