HANDLING SMALL AMOUNTS OF MATERIAL: THE CENTRIFUGE 283 
made. A stock should be provided so that a number are always 
on hand. It will be found convenient to have sedimenta¬ 
tion tubes of different diameters, to permit varying amounts 
of liquid being used. Similarly constructed smaller tubes of 
thinner wall can be made to fit inside the ordinary “sputum” 
tubes usually furnished with the centrifuge. 
Once having become accustomed to using this instrument, the 
worker in microchemistry will find that the two-speed centrifuge 
is an almost indispensable instrument, which will enable him to 
meet with ease all sorts of problems involving the separation of 
solids and liquids that would otherwise tax his patience and 
ingenuity. 
Especially to be recommended are electrically driven centri¬ 
fuges provided with protecting hoods. 
When dealing with relatively large volumes of liquid the usual 
conical sedimentation tubes, shown in Fig. go, will prove useful, 
but since it is usually the sediment which is to be subjected to 
examination or analysis, and rarely the liquid, it will be found 
more convenient to employ tubes drawn down to a fairly long 
pointed end which may be cut off with a file scratch just above 
the sediment, thus permitting easy access to the solids thrown 
out from suspension. When properly drawn down, tubes of this 
form can be used several times by simply sealing the end; the 
tubes are centered and held in the aluminum carriers by means 
of perforated corks. 
Occasionally tubes with removable parts will be found to be 
convenient; the best forms are those devised by T. W. Richards^ 
for the separation of small quantities of crystals from mother 
liquor. The construction and method of employment of these 
tubes will be readily understood by reference to Fig. 147. 
When one of the modern large electric laboratory centrifugal 
machines ^ is available very minute amounts of suspended matter 
may be separated from large volumes of liquid with great ease. 
The most convenient form of apparatus for this purpose con¬ 
sists in fitting a Squibb’s separatory funnel with a stopcock of 
^ Richards, J. Amer. Chem. Soc., 27 (1905) 104. 
* As for example the Bausch and Lomb Precision Centrifuge. 
