THE HALOGENS 499 



convenient to take advantage of the reactions between phosphorus, 

 the halogens, and water, when the latter is present in small quantity 

 (otherwise the halogen acids formed are dissolved by the water), and 

 the halogen is gradually added to the phosphorus moistened with 

 water. Thus if red phosphorus be placed in a flask and moistened with 

 water, and bromine be added drop by drop (from a closed funnel with 

 a glass tap), hydrobromic acid is abundantly and uniformly dis- 

 engaged. 76 Hydrogen iodide is prepared by adding 1 part of common 

 (yellow) dry phosphorus to 10 parts of dry iodine in a glass flask. On 

 revolving the flask, reaction (light and heat are evolved) proceeds quietly 

 between them, and when the mass of the iodide of phosphorus which is 

 formed has cooled, water is added drop by drop (from a funnel with a 

 tap) and hydrogen iodide is evolved even without the aid of heat. 

 These methods of preparation will be at once understood when it is 

 remembered (p. 462) that phosphorus chloride gives hydrogen chloride 

 with water. It is exactly the same here the oxygen of the water 

 passes over to the phosphorus, and the hydrogen to the iodine ; for 

 example, PI 3 + 3H 2 O =PH 3 O 3 + 3HI. 77 



In a gaseous form hydrobromic and hydriodic acids are closely 



equilibria in all their complexity is naturally very great, but it falls in the background 

 before the primary interests of discovering practical methods for the isolation of sub- 

 stances, and the means of employing them for the requirements of man. It is only after 

 the satisfaction of these requirements that interests of the other order arise, which 

 themselves reflect the interests of the primary nature. For these reasons, whilst con- 

 sidering it opportune to point out the theoretical interests of chemical equilibria, the 

 chief attention of the reader is directed in this work to the primary chemical interests. 



76 Hydrobromic acid is obtained by the action of bromine on paraffin heated to 180. 

 Gustavson proposed preparing it by the action of bromine (it is best if it be added in 

 drops together with traces of aluminium bromide) on anthracene (a solid hydrocarbon 

 from coal tar). Balard prepared it by passing bromine vapour over moist pieces of 

 common phosphorus. The liquid tribromide of phosphorus, directly obtained from 

 phosphorus and bromine, also gives hydrobromic acid when treated with water. Bromide 

 of potassium or sodium, when treated with sulphuric acid in the presence of a piece of 

 phosphorus, also gives hydrobromic acid, but hydriodic acid is decomposed by this 

 method. In order to free hydrobromic acid from bromine vapour it is passed over moist 

 phosphorus and dried either by phosphoric anhydride or calcium bromide (calcium 

 chloride cannot be used, as hydrochloric acid would be formed). Neither hydrobromic 

 nor hydriodic acids can be collected over mercury, on which they act, but they may be 

 directly collected in a dry vessel by letting the gas-conducting tube pass down to the 

 bottom of the vessel, both gases being much heavier than air. 



77 But, generally, more phosphorus is taken than is required for the formation of PI-, 

 because, otherwise, a portion of the iodine is distilled over. If less than one-tenth part 

 of iodine be taken, then much phosphonium iodide, PH 4 I is formed. This proportion 

 was established by Gay-Lussac and Kolbe. Hydriodic acid is also prepared in many 

 other ways. Bannoff dissolves two parts of iodine in one part of a previously-prepared 

 strong (sp. gr., T67) solution of hydriodic acid, and pours it on to red phosphorus in a 

 retort. Personne takes a mixture of fifteen parts of water, ten of iodine, and one of red 

 phosphorus, which, when heated, disengages hydriodic acid mixed with iodine vapour, 

 which is removed by passing it over moist phosphorus (Note 76). It must be remembered, 



K K 2 



