THE HALOGENS 505 



convenient to take advantage of the reactions between phosphorus, 

 the halogens, and water, the latter being present in small quantity 

 (otherwise the halogen acids formed are dissolved by it) ; 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 tap funnel), hydro- 

 bromic acid is abundantly and uniformly disengaged. 76 Hydrogen 



detail (of the quantity of water, temperature, pressure, mass, &c.) The study of 

 practical methods of preparation is therefore one of the paths for the study of chemical 

 mechanics. The reaction of iodine on phosphorus and water is a case like that men- 

 tioned in Note 74, and the -matter is here further complicated by the possibility of the 

 formation of the compound PH 3 with HI, as well as the production of PI 2 , Pis, and the 

 affinity of hydriodic acid and the acids of phosphorus for water. The theoretical 

 interest of equilibria in all their complexity is naturally very great, but it falls into the 

 background in presence of the primary interest of discovering practical methods for the 

 isolation of substances, 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 in their turn must exert an influence on the former. For these reasons, 

 whilst considering it opportune to point out the theoretical interest of chemical 

 equilibria, the chief attention of the reader is directed in this work to questions of 

 practical importance. 



76 Hydrobromic acid is also obtained by the action of bromine ou paraffin heated 

 to 180, Gustavson proposed to prepare it by the action of bromine (best 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. Bro- 

 mide of potassium or sodium, when treated with sulphuric acid in the presence 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 hydrobromio 

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

 may be directly collected in a dry vessel by leading the gas-conducting tube to the 

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

 (1889) propose to prepare HBr directly from bromine and hydrogen. For this purpose, 

 pure dry hydrogen is passed through a flask containing boiling bromine. The mixture 

 of gas and vapour then passes through a tube provided with one or two bulbs, which 

 is heated moderately in the middle. Hydrobromic acid is formed with a series of flashes 

 at the part heated. The resultant HBr, together with traces of bromine, passes into a 

 Woulfe's bottle into which hydrogen is also introduced, and the mixture is then carried 

 through another heated tube, after which it is passed through water which dissolves the 

 hydrobromic acid. According to the method proposed by Newth (1892) a mixture of 

 bromine and hydrogen is led through a tube containing a platinum spiral, which is 

 .heated to redness, after the air has been displaced from the tube. If the vessel con- 

 taining the bromine be kept at 60, the hydrogen takes up almost the theoretical amount 

 icf bromine required for the formation of HBr. Although the flame which appears in the 

 neighbourhood of the platinum spiral 'does not penetrate into the vessel containing the 

 bromine, still, for safety, a tube filled with cotton wool may be interposed. 



Hydriodic acid is obtained in the same manner as hydrobromic. The iodine is heated 

 in a small fiask, and its vapour is carried over by hydrogen into a strongly heated tube. 

 The gas passing from the tube is found to contain a cdnsiderable amount of HI, together 

 with some free iodine. At a low red heat about 17 p.c of the iodine vapour enters 



