METHODS FOR HANDLING SMALL AMOUNTS OF MATERIAL 235 



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. 76, 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 1 

 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. 124. 



When one of the modern large electric laboratory centrifugal 

 machines 2 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 



1 Richards, J. Amer. Chem. Soc., 27 (1905) 104. 



2 As for example the Bausch and Lomb Precision Centrifuge. 



