TESTING OF THE DIALYSING TUBES 



161 



is reached the test-tube is brought to the edge of the 

 flame, and the boiling is continued at the middle 

 height of the flame (see fig. 8). In this way it is 

 possible to carry out the boiling continuously and 

 energetically, so that the liquid travels over more than 

 half of the test-tube, without any danger of over- 

 boiling. Not for a single moment should the atten- 

 tion be allowed to wander from this process, for 



FIG. 8. 



(a) Test-tube holder; (b) boiling-stick. 



everything depends upon the accuracy of the 

 operation. If the ebullition is too weak, then under 

 certain circumstances the reaction may fail altogether, 

 while, if the rate of ebullition differs in the different 

 tests, we get a difference in the intensities of the 

 colorations. The results, in short, are inaccurate. 

 After the lapse of half an hour a comparison of the 



I I 



