26 § 36. ANALYTICAL MANIPULATION. 



alcohol, and ether, for organic matters. As the manner of 

 making the solution is described in each case, when spe- 

 cial directions are necessary, but little need be said on the 

 subject here. As a general rule, heat increases the sol- 

 vent power of the dissolving agents to a considerable ex- 

 tent, and hence it should always be applied, unless the 

 solution is very easily accomplished without, or unless di- 

 rections are given to the contrary. Time is often an im- 

 portant element in effecting solution, and hence long con- 

 tinued digestion at a moderately high temperature may 

 be useful, or even necessary. A great excess of strong 

 acid in a solution to be analyzed often causes much 

 trouble ; hence, as little acid as possible should be used, 

 and in case a large quantity has been added to the sub- 

 stance, it should, in most cases, be removed subsequently 

 by evaporation almost to dryness. 



Unless a substance is readily and completely soluble, it 

 is essential that it should be as finely divided as possible, 

 and, to this end, it should be ground to a fine powder in 

 a porcelain mortar, or, better still, an agate one. 



In order to reduce a substance to a sufficiently fine 

 powder, it is sometimes necessary to levigate it, Avhich 

 means simply to grind it in the agate mortar with the 

 addition of water enough to make a thin paste, until no 

 grittiness can be felt under the pestle, nor any grating 

 sound heard. Then rinse the contents of the mortar 

 into an evaporating dish, dry the substance thoroughly 

 over the water-bath, and mix the dry residue together 

 carefully by further grinding in the mortar. 



In making a solution for qTiantitative purposes, when 

 the loss of even a minute part of the substance would 

 impair the accuracy of the results obtained, if the mixture 

 of substance and solvent is to be boiled, or if the sub- 

 stance is a carbonate, and is to be treated with an acid, it 

 is best to operate in a flask placed on its side, or with its 

 mouth loosely stoppered by a small funnel, or in a beaker 



