16 NOTES ON LABORATORY MANIPULATIONether evaporates so rapidly that a preparation wet with it may be
dried by a very few minutes' exposure to the air.
Alcohol and ether are both expensive and should be used spar-
ingly. They can be used most effectively as follows: After all
the water possible has been drained from the preparation, transfer
the latter to an evaporating dish and pour over it enough alcohol
to moisten it thoroughly; stir it with a spatula until the alcohol
has penetrated to every space between the crystal grains, then
pour off, or drain off, the alcohol and treat the preparation in like
manner with another portion of fresh alcohol. After that wash
it once or twice with ether in exactly the same way. If the
preparation is washed on the filter, drain off the water as thor-
oughly as possible, stop the suction, add just enough alcohol to
moisten the whole mass, and after letting it stand a few moments
drain off the liquid completely. Apply a second portion of alcohol
and portions of ether in the same manner.
- PULVERIZING
In chemical reactions in which solid substances are involved the
action is limited to the surface of the solid, and for this reason it is
evident that it must be much slower than reactions which take
place between dissolved substances; it is also evident that, the
more finely powdered a solid substance, the greater is its surface,
and therefore the more rapidly it will react.
For grinding any quantity of a substance a large porcelain
mortar (say 8 inches in diameter) with a heavy pestle is preferable
to the small mortars usually supplied in the desks. One or more
such mortars is placed in the laboratory for general use.
If a hard substance can be obtained only in large pieces, it should
first be broken with a hammer, then crushed into small particles
in an iron or steel mortar, after which it should be ground in the
porcelain mortar. In the final grinding it is often advisable to sift
the fairly fine from the coarser particles, then to finish grinding
the former by itself and to crush and grind the coarser particles
apart. - NEUTRALIZING
Various indicators are used to determine whether a solution is
acidic or basic. For example, litmus is red in the presence of acid,
blue in the presence of a base, and of an intermediate purple tint