28 § 37. AXALYTICxVL MANIPULATION. 



orated, the operation may be performed at first directly 

 over the lamp ; but in quantitative work the evaporation 

 should be completed on the water-bath in all cases ; if the 

 original quantity of the solution is small, it is better to 

 conduct the wliole evaporation on the water-batli. 



If the evaporation is connected with quantitative work, 

 the dish should never be more than three-fourths filled, 

 and the solution should not be allowed to boil at any time 

 ia an oj)en vessel ; evaporation will, however, proceed 

 quite rapidly in a flask placed partly on its side, and in 

 this case gentle boiling may be allowed. 



Unless the evaporation is performed in a room set apart 

 for the work, and entirely free from dust, solutions should 

 'be kept covered with filter-j^aper during the operation ; 

 the paper should be supported by glass rods, or a glass 

 triangle, laid over the dish in such a manner that it cannot 

 conic in contact with the liquid ; if the solution is strong- 

 ly acid, the paper should have been well washed with 

 acid, as directed for washing filters § 39, a / otherwise, 

 drops of acid, that have condensed on the glass rods 

 and come in contact with the paper, may fall back into 

 the liquid and carry with them inorganic substances that 

 were dissolved out of tlic paper. 



To prevent the salts in solution from being deposited 

 on the sides of the dish above the liquid, and even over 

 the edge, smear the rim of the dish, just below the edge 

 on the inside, with the thinnest possible coat of tallow. 

 Or, fit the dish in a little jacket of fire-clay, in sucli a 

 manner that the part of it above the liquid shall be kept 

 very hot. Or, turn the crucible on its side, and apply 

 the flame of the lamp just above the surface of the liquid. 



h. When, as is often the case in agricultural analysis, 

 potassa or soda is to be estimated in a solution containing 

 a large quantity of amjnoniacal salts, and from which 

 these salts are to be removed by evaporation to dryness 

 and ignition, Fresenius recommends to evaporate the so- 



