FORMATION OF ORGANIC COMPOUNDS. 119 



two acids with the base; K, K^ the heats disengaged by the 

 action of the salt formed by the other acid. 



Theorem IX. Similarly the reciprocal action of bases on the 

 salts which they form with the same acid 



K'i - K' = M - ML 



Theorem X. The reciprocal action of the four salts formed by 

 two acids and two bases is expressed by the formula 



K! - It = (M - M') - (M L - M'O, 



K being the heat liberated, when the solutions of two salts with 

 different acids and bases (potassium sulphate and sodium nitrate) 

 are mixed, and Kj the heat liberated when the reciprocal pair 

 are mixed (sodium sulphate and potassium nitrate). This 

 theorem enables us to determine the double saline decompositions 

 which are effected in solutions, when two salts of the same acid 

 or the same base are unequally decomposed by the same quantity 

 of water, which happens in the case of weak acids and bases, 

 and the metallic oxides. 



3. Theorems on the formation of Organic Compounds. 



The heat of formation of organic compounds, by means of their 

 elements, cannot be directly measured, but it may be calculated 

 by the aid of various theorems, which follow from the second 

 principle. 



Theorem I. Difference between the heats of formation from the 

 elements. Let there be two distinct systems of compounds, formed 

 from their elements, carbon, hydrogen, oxytfen and nitrogen, or 

 from very simple binary compounds, such as water, carbonic acid, 

 carbonic oxide, ammonia j the difference between the heat of forma- 

 tion of the first system and that of the second is equal to the heat 

 liberated when one of the systems is transformed into the other. 



It is in this way that the heat of formation of bodies belong- 

 ing to the cyanogen series has been measured. 



Theorem II. Difference between the heats of combustion. 

 The heat of formation of an organic compound by its elements is 

 the difference between the sum of the heats of total combustion of 

 its elements by free oxygen and the heat of combustion of the com- 

 pound with formation of identical products. 



It is in virtue of this principle that most of the heats liberated 

 by the formation of organic compounds and their reciprocal 

 transformations have been obtained. 



Theorem III. Conversely, the heat of combustion of a body 

 formed of carbon, hydrogen, oxygen and nitrogen, is calculated by 

 means of its heat of formation. It is sufficient to find the sum 

 of the quantities of heat liberated when the carbon and hydrogen 

 supposed free, which enter into the composition of this body, are 

 changed into water and carbonic acid, and to deduct from this 

 sum the heat of formation. 



