19O SOURCES OF ERROR (IN THE PROCESS) 



always some which allow the passage of albumen. 1 

 Or they may become useless subsequently, generally 

 becoming permeable to albumen. This, however, 

 only occurs when they are handled improperly. 

 They must not be cleaned with a rough brush, nor 

 must they be boiled for too long a time. Tubes 

 may become impermeable to peptones through over- 

 boiling, so that, though they should be thoroughly 

 washed, they should be boiled but slightly. They 

 must be kept in sterilized water with a thick layer of 

 toluol (see p. 162), and must never be left for a long 

 time unemptied of their contents. 



A great source of error which is, however, 

 impossible with proper manipulation, is due to 

 tubes being insufficiently cleaned. The result of this 

 is, that the wall of the tubes will contain traces of 

 substances, which react with ninhydrin if sufficiently 

 concentrated. They may be present in such minute 

 quantities as to be unable of themselves to produce 

 a coloration ; yet they will, when added to the 

 analogous substances that are present in the serum, 

 convert a negative reaction into a positive one. 

 Therefore the utmost possible care must be exercised 

 in the manipulation of the tubes. 



18 We have recently observed up to So per cent, of useless 

 tubes. It would be very desirable if a dialysing tube could 

 be produced which was, at the least, indubitably impermeable 

 to albumen. 



