204 Dr. Macneven's Exposition of [Sept. 



second loses half its carbonic acid in a moderate heat, whereas 

 the first half is retained even in a red one. The carbonate con- 

 sists of one atom base and one atom acid, but the bicarbonate of 

 one atom base and two atoms acid. The union of one atom 

 base and one atom acid is energetic, but that of one atom base 

 and two atoms acid is easily broken. 



21. When a body has the property of uniting with various 

 closes of oxygen, we are best enabled to determine the number 

 of atoms which constitute the compounds. Thus manganese 

 unites with four doses of oxygen, and if the manganese be repre- 

 sented by 100, the oxygen of each respective oxide is found to 

 be 14, 28, 42, 56. These numbers are in arithmetical progres- 

 sion, having the common difference 14, and are to each other as 

 1, 2, 3, 4. Hence, the first oxide is composed of one atom 

 manganese and one atom oxygen ; the second of one manoanese 

 -h two oxygen ; the third, of one manganese + three oxygen ; 

 and the fourth, of one manganese and four oxygen. It is 

 observable that the fourth proportion of oxygen is very easily 

 separated, the second and third are less and less so, and the 

 separation of the first, or the total decomposition of the oxide is 

 extremely difficult. 



In like manner as mercury unites with two doses of oxygen and 

 forms two oxides, the first composed of 100 mercury and 4 oxy- 

 gen, and the second of 100 mercury and 8 oxygen ; the first 

 must be a compound of one atom mercury + one atom oxygen, 

 and the second of one atom mercury + two oxygen. 



There are two oxides of iron ; the first composed of 100 iron 

 and 28 oxygen ; the second, of 100 iron and 42 oxygen. Now, 

 as 28 : 42 :: 2 : 3, it follows that the first is composed of 100 

 iron and 2 atoms oxygen, and the second of 100 iron and 

 3 atoms oxygen ; and we may infer from analogy that there is a 

 lower oxide consisting of 100 iron and 1 atom oxygen. 



22. When once we know the gross number of atoms and the 

 amount of each kind in a compound, their proportional weight 

 is easily determined. Tims, if black oxide of mercury be com- 

 posed of one atom mercury and one atom oxygen, and that 100 

 mercury combine with 4 oxygen, which experiment proves it 

 does ; then the weight of an atom of mercury is to the weight of 

 an atom of oxygen as 25 to 1 ; and the weight of an atom of 

 black oxide will be represented by 25 + 1 = 26. 



23. If water be composed of one atom oxygen and one atom 

 hydrogen, and if the weight of the oxygen in water is to that of 

 the hydrogen in water, as 8 : 1, then it follows that an atom of 

 oxygen is eight times as heavy as an atom of hydrogen. 



24. Such is the method of determining the weight of an atom 

 of the different substances best known to us by accurate experi- 

 ment. It enables us to calculate the proportions of the consti- 

 tuents of all compound bodies. It is a standard with which to 

 compare om- experiments and try their accuracy. Those that 



