126 ELEMENTS OF LABORATORY WORK 



from this fact that a peculiar closeness of contact is necessary 

 before chemical change can occur ; a supposition which is 

 strengthened by all chemical research, and especially by the fact 

 that solids, which may exist without change side by side, even 

 when finely divided, for any length of time, may be induced 

 by great pressure to combine chemically. 



83, Chemical Combination always takes place between 

 Definite Quantities; consequently each Elementary Substance 

 has a Specific Value in Chemical Exchange, The element 

 magnesium, when placed in a solution of silver nitrate, is dis- 

 solved, and at the same time the element silver is precipitated. 

 If a known quantity of magnesium is added to silver nitrate, 

 sufficient in quantity to completely dissolve it, the quantity 

 of silver precipitated will always be found to have the same 

 ratio to the quantity of magnesium taken. 



In the same way the element iron is dissolved by a 

 solution of copper sulphate, while the element copper is 

 simultaneously precipitated. For the same quantity of iron 

 dissolved, the same quantity of copper is always precipitated. 

 Similar observations may be made with other substances, 

 and it will be always found that definite invariable quantities 

 of various elements are, in cases of chemical action, equivalent 

 to one another. The relation existing between the quantities 

 taking part in the same chemical change never varies. A 

 given compound is always formed of the same constituents 

 united in the same relative quantities, whatever may have 

 been the conditions under which it was formed. 



Weigh a porcelain crucible, and add some pure magnesium 

 in small pieces of ribbon, and weigh again. Add to the 

 known quantity of magnesium in the crucible a quantity of 

 silver nitrate solution, sufficient to react completely with the 

 quantity of magnesium present. Preliminary experiments 

 will have indicated the quantity needed. Constant stirring with 

 a short glass rod is advisable. The temperature of the solution 

 will be noticed to rise during the reaction. After the mag- 

 nesium has disappeared the solution is gently heated. The 

 silver, which has been precipitated, is separated from the 

 excess of silver nitrate by decanting through a filter and 



