GENERAL OBSERVATIONS 23 



drying- samples preparatory to grinding for analysis it is best to 

 stop the process as soon as the materials can be pulverized. In 

 general, samples should be dried only to determine water, and 

 the analytical processes should be performed on the undried 

 portions. It is not necessary, as a rule, to dry samples of mineral 

 fertilizers in an inert atmosphere, such as hydrogen or carbon 

 dioxid. Drying in vacuo may be practiced when it is desired 

 to secure a speedy desiccation or one at a low temperature. 



30. Official Methods. The official agricultural chemists direct, 

 in the case of potash salts, sodium nitrate, and ammonium 

 sulfate, to heat from one to five grams at about 130 until the 

 ueight is constant. 11 The loss in weight is taken to represent the 

 water. For all other fertilizers heat two grams, or five grams 

 if the sample be very coarse, for five hours in a steam bath. The 

 international commission prescribes heating to constant weight 

 at 100, using 10 grams of material. Substances containing gyp- 

 sum are dried for three hours. Potash salts are dried in harmony 

 with the regulations prescribed by the potash syndicate at 

 Leopoldshall, Stassfurt. 



In the German stations in the case of untreated phosphates 

 and bone-black the moisture is estimated at from 105 to 110. 

 Samples which lose ammonia should have the weight of ammonia 

 given off at that temperature, determined separately. 



31. General Observations. For purposes of comparison it would 

 be far better to have all contents of moisture determined at the 

 boiling point of water. While this varies with the altitude and 

 barometric pressure yet it is quite certain that the loss on drying to 

 constant weight at all altitudes is practically the same. Where 

 the atmospheric pressure is diminished for any cause the water 

 escapes all the more easily. This, practically, is a complete com- 

 pensation for the diminished temperature at which water boils. 



Only in the case where free sulfuric or phosphoric acid is present 



would this method be ineffective. The highly hygroscopic nature 



of these acids in a concentrated state renders desiccation at such a 



temperature practically impossible. In such cases a weighed ex- 



11 Division of Chemistry, Bulletin 46, 1899 : n. 



