WATER CONTENT AND ASH OF PLANTS 157 



temperature of 100 C., the boiling-point of water. This 

 converts all of the water in the material into steam, 

 which is then expelled. A water-oven, shown in Fig. 

 62, has double walls, the space between the walls being 

 partially filled with water, which is kept boiling by means 

 of a gas burner placed below the oven. The substance is 

 weighed in a suitable dish and then dried in the water- 

 oven until the weight is reasonably constant, the loss of 

 weight being considered water. 



The determination of water in foods, although appar- 

 ently simple, is a difficult and troublesome chemical pro- 

 cess because many foods, when heated to 100 C., suffer 

 changes, and give off . volatile organic compounds along 

 with the water ; or the organic matter may undergo a 

 change in composition, as oxidation. For determining 

 the absolute moisture content of foods, the chemist em- 

 ploys a drying bath of different pattern from that shown, 

 and the material is dried in a current of some neutral gas, 

 as hydrogen, to prevent oxidation of the 

 substance. All of the dishes in which 

 the substances are placed, during anal- 

 ysis, are dried and cooled in desiccators 

 out of contact with air, so as to remove 

 all traces of hydroscopic moisture. The 

 weighings are made on analytical bal- Fig. 64. Desiccator, 

 ances which are scales of extreme accuracy (see Fig. 63). 

 The determination of water is one of the most difficult 

 parts of the analysis of plant or animal substances. 



206. Dry flatter. The dry matter of a material is the 

 portion which is left after all the water has been re- 



