30 Estimations occurring in Agricultural A nalysis [45, 46 



final washings by means of a glass slip, as described in para- 

 graph 12 (k). 



45. Ignition and Weighing. — Dry the precipitate, and 

 prepare it for weighing as follows : Remove as much of the 

 precipitate as possible from the paper to a weighed platinum 

 dish, keeping it well on one side of the dish. Burn the filter 

 rapidly on a spiral of platinum wire, and whilst still black drop 

 it into the dish, so that it may lie in direct contact with the 

 platinum and not on top of the calcium oxalate. Place the 

 dish on an Argand with the flame turned low, and gradually 

 turn it up until, in about half-an-hour, it is just below a red 

 heat. Now watch the filter paper carefully for a few minutes. 

 If it should begin to burn, or if sparks appear on its surface, 

 the flame must be lowered. If in the course of half-an-hour 

 the paper is still perfectly black, the flame must be turned up ' 

 a little. When the right temperature is obtained in this way 

 the paper will gradually crumble down to a light-grey coloured 

 mass. As soon as all the black of the filter has gone, which 

 should take place in about forty minutes from commencing, 

 the dish may be cooled down in a desiccator and weighed. 



A very little practice is sufficient to enable the analyst to 

 hit the right temperature for this reaction, but a second weigh- 

 ing is generally advisable after heating at the same temperature 

 as before for ten minutes longer. The second weighing 

 should not be more than half a milligram different from the 

 first. 



N.B. — It is frequently recommended that the CaCO s obtained in this 

 manner be moistened with ammonium carbonate solution, and re-heated 

 before weighing. This is to bring back any CaO which may have been 

 formed to CaC0 3 . Any additional accuracy gained by this method is 

 more than made up for by the chances of loss by spirting. 



46, When the weight of the CaC0 3 has been verified, the 

 dish may be placed in a Fletcher muffle furnace and kept at 



