40 § 41. ANALYTICAL MANIPULATIOX. 



wards to be exposed with the precipitate, and weighed, 

 either between two watch-glasses with ground edges and 

 fitting well together, or in a stoppered glass tube ; after 

 careful drying with the precipitate, it is again w^eighed in 

 the same manner. It should then bo dried an hour 

 longer and weighed again, and this should be repeated 

 until a constant weight is obtained. Swedish filter-paper 

 or washed filters should always be used in this operation. 



c. The substance that has been dried or ignited, and is 

 to be weighed, should always be allowed to cool under a 

 bell-glass over concentrated sulphuric acid, or in the des-' 

 locator more commonly used for this purpose ; this desic- 

 cator consists simply of a short and wide glass cylinder, 

 with a ground edge upon Avhicli a ground glass plate will 

 fit closely, particularly if the edge is smeared with a lit.lo 

 tallow. 



The pair of w^atch-glasses containing the dried filter, 

 or the crucible with the ignited precipitate, rests on a tri- 

 angle in the cylinder over fused calcic chloride, with 

 which the bottom is covered. 



No object^ should be weighed until it is entirely cold. 



/. Platinum vessels, after having been heated by gas, 

 should be rubbed with a little sand on the moistened fin- 

 ger. The sand should be fine, and all its grains should 

 be rounded. The crucible should also be cleaned from 

 time to time by fusing a little potassic bisulphate in it. 

 The crucible should be supported over the lamp on stout 

 platinum wire, which is stretched from side to side of a 

 larger iron-wire triangle, in such a manner as to make a 

 second triangle inside of, and about C mm. smaller than, 

 the iron triano^le. 



41. 



del 



MEASURING AND DIVIDING SOLUTIONS. 



41, For these purposes graduated pipettes and cylii 

 rs, and ' |^, ' !„, and 1 litre flasks are used. 



