I6O TESTING UF THE D1ALYSING TUBES 



water present, while on the other hand mould may 

 easily appear. Again, the boiling-sticks should never 

 be dried at too high a temperature, otherwise they may 

 turn brown, and in that case they give off a brown 

 colouring matter during boiling, and thus render 

 an exact reading impossible. They must never be 

 touched with the hands, but should always be placed 

 in the test-tubes by means of forceps. 



The process of boiling is now started, and the 

 manner in which this is carried out is of the greatest 

 importance. Boiling must be intensive; at the same 

 time every precaution must be taken to avoid the 

 slightest spilling, as also to prevent uneven evapora- 

 tion. When all the liquids to be tested have been 

 boiled, we must assure ourselves that they are at the 

 same level in all the test-tubes. It is best to use 

 large test-tubes upon which the volume of 10 c.c. 

 is conspicuously marked. It is then easy to ascer- 

 tain whether the very important point of even boiling 

 has been accurately carried out. 



The test-tube is first held by means of a holder in 

 the centre of a Bunsen burner, the flame of which 

 must be a full one. One then watches carefully for 



/ 



the moment when the first bubbles of gas appear on 

 the sides of the test-tube, which only takes a few 

 seconds, and calculating from this moment one boils 

 for exactly one minute. After ten to fifteen seconds a 

 vivid ebullition is observed, and as soon as this point 



