280 ELEMENTARY GENERAL SCIENCE CHAP. 



means of nitric acid, which is a powerful oxidising agent (i.e., a 

 substance which readily gives up oxygen to other bodies). 



EXPT. 288. Place some phosphorus, either variety, in an 

 evaporating basin, add some nitric acid, and warm gently 

 in a draught cupboard. Notice the energetic action and the 

 evolution of red fumes (a sign that the nitric acid is giving up 

 oxygen). Evaporate nearly to dryness, then add a little more 

 nitric acid and evaporate down again. You thus obtain a 

 gelatinous product, which cools to a hard glassy mass. 



The phosphoric acid so obtained is a crystalline solid, which 

 dissolves in water. Like the other acids already examined, it 

 may be neutralised by the addition of caustic soda, and a salt 

 obtained. The salts obtained from phosphoric acid are known 

 as phosphates. Owing to the presence of three hydrogen atoms 

 in the molecule of phosphoric acid, we may obtain three sets of 

 salts in which one, two, or three hydrogen atoms are replaced ; 

 thus with sodium we may obtain trisodium phosphate, Na 3 PO 4 ; 

 hydrogen disodium phosphate, HNa 2 PO 4 ; and dihydrogen sodium 

 phosphate, H 2 NaPO 4 . Salts like the two latter, in which all the 

 hydrogen is not replaced are known as acid salts; those which, 

 like the first, contain no acid hydrogen are termed normal salt*. 



Phosphorous Oxide. Other oxides of phosphorus, however, 

 exist besides the pentoxide, and of these the most important is 

 that known as phosphorus trioxide, phosphorous oxide, or phos- 

 phorous anhydride, P^g. This results, to a small extent, when 

 phosphorus burns in air, the quantity increasing when the supply 

 of air is insufficient. It is a white solid with a garlic-like odour, 

 and it differs greatly from the pentoxide in its action on water, 

 as it is only very slowly dissolved, forming an unstable acid, 

 phosphorous acid, H 3 P0 3 , the salts of which are called phosphites. 



Compounds of Phosphorus and Hydrogen. By the action 1 

 of phosphorus on caustic soda, a most remarkable product 

 results, and the following experiments should be done to obtain 

 and study this compound. 



EXPT. 289. Fit up apparatus in the manner indicated in 

 Figure 134. In the retort place some pieces of phosphorus 

 and a strong solution of caustic soda. Then completely 

 displace the air from the apparatus by means of a current ' 

 of coal gas. When this is done the retort may be heated. 

 Observe that action goes on inside and that a gas with an 



