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 evaporation. 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 ascertain 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
