94 



CHEMISTRY FOR AGRICULTURAL STUDENTS 



phosphorus or phosphoretted hydrogen is often liberated. It 

 is a gas of an odour even more offensive than sulphuretted 

 hydrogen, and to it the smell of rotting fish is partly due. Iron 

 is often impure with phosphide of iron. Such iron, when 

 treated with dilute sulphuric acid, yields hydrogen mixed with 

 phosphoretted hydrogen (compare with sulphur, p. 68), 

 and this may be detected in the smell of the escaping gas, 

 and the colour of its flame when ignited. The composi- 

 tion of phosphoretted hydrogen is expressed by the formula 

 PH3. 



Touch a fragment of dry phosphorus with a glass rod which 

 has been heated till it feels warm to the hand. Note that even 

 at this moderate temperature the phosphorus ignites, clouds 

 of oxide of phosphorus being produced. Hold another freshly 

 cut fragment on the point of a penknife. Note that, without 

 ignition, oxidation is proceeding. Examine it in the dark. 

 Note that this slow oxidation is accompanied h-^ phosphorescence. 



On account of its ready combustibility, phosphorus is used on the heads 

 of kicifer matches to promote their ignition. A mixture of phosphorus and 

 nitre is employed which, being explosive, is ignited by mere friction. 

 "Safety matches " are tipped with the oxidising agent, while the combustible 

 substance, a mixture of amorphous phosphorus and the sulphide of anti- 

 mony, is smeared on the box. To ignite 

 such matches, the match must, therefore, be 

 rubbed on the box. Amorphous phosphorus 

 is an allotropic modification of phosphorus 

 not spontaneously inflammable. It is formed 

 when phosphorus is heated to 240° C. in 

 absence of air. 



Fill a dry bell jar resting on a dry 

 plate with dry oxygen gas (supplied), 

 and insert a deflagrating spoon con- 

 taining some ignited dry phosphorus. 

 Note the formation of white phos- 

 phoric anhydride. Collect this on a 

 spatula and plunge into cold water. 



Fig. 39. 



