130 ELEMENTS OF LABORATORY WORK 



required to give the maximum quantity of precipitate is ascer- 

 tained. We ought, in fact, to proceed so as to obtain at each 

 operation narrower limits. Care is needed in reading the 

 height of the column of the solution which is in the burette. 



It will be found that equal quantities of the silver nitrate 

 solution require equal quantities of the sodium chloride solution. 

 Measurements must be made accurately, and under the same 

 conditions. Equal quantities of the same solution, however, 

 contain equal quantities of the dissolved substance, as has 

 been shown by observations of diffusion. 



In addition to this proof of the equality of the reacting 

 quantities, we have an indirect one in the fact that the 

 quantities of precipitate are exactly equal when equal quanti- 

 ties of solution are used. The precipitates given in two of 

 the previous operations are separated, by nitration through 

 papers of which the weights have been taken after drying ; 

 washed with water, to remove from them any other kind of 

 matter , and then thoroughly dried. The quantities thus 

 measured will be found to be equal. It is easy to calculate, 

 from the method adopted, the relative quantities of silver 

 nitrate and sodium chloride which react with one another. 



It has been taken for granted in this operation that the 

 precipitate yielded by " the reacting bodies is insoluble in 

 water. This may be shown to be true in the manner pre- 

 viously described. It has also been taken for granted that 

 the cessation of the precipitation marks the end of the chemi- 

 cal change in question ; or, in other words, that the addition of 

 further quantities of the one or the other reagent is followed 

 by no further change. It is present, as any other inert body 

 might be, in an unaltered condition. This may be proved 

 by those processes of fractional distillation and solidification 

 which have been already described, or by further chemical 

 investigations. 



86, Some Conditions of Chemical Change, Two kinds of 

 Complexity of Matter. We have learnt that a complex body 

 may be separated into parts by making use of any difference 

 in the physical properties of these parts. For example, a 

 difference of solubility in water allows us to separate a more 



