SUBSTANCE AND FORM. 



that the groups of their atoms (the molecules) 

 are of different sizes and perhaps of different 

 arrangements. 



There are three kinds of tartaric acid, all 

 containing the same proportions of carbon, 

 hydrogen, and oxygen. Yet they all act differ- 

 ently upon light. A solution of one turns the 

 plane of polarisation to the right, another turns 

 it to the left ; while the third lets it pass through 

 straight. 



Dextrose is a kind of sugar which owes its 

 name to the fact that it turns the plane of 

 polarisation to the right. It is found in sweet 

 fruit, with an equivalent quantity of another 

 sugar, which is called levulose, because it turns 

 the plane of polarisation to the left. Yet both 

 of these sugars consist of molecules each con- 

 taining six atoms of carbon, six of oxygen, and 

 twelve of hydrogen. 



So oil of turpentine, oil of lemons, and oil of 

 lavender all have as chief constituent a sub- 

 stance whose molecules contain ten atoms of 

 carbon and sixteen of hydrogen ; but in each 

 of them their substance differs^from the similarly 

 composed substances in the others. 



It is important, therefore, to remember that 

 nature's cookery can produce very different 

 dishes out of the same ingredients taken in the 

 same proportions and so little altered that they 

 can be recovered in the same state in which 

 they were before the cooking began. 



o 209 



