THE HALOGENS 498 



and is distinguished by its poisonous properties, so that in working with 

 the acid a strong draught must be kept up, to prevent the possibility of 

 the fumes being inhaled. The non-metals do not act on hydrofluoric 

 acid, but all metals with the exception of mercury, silver, gold, and 

 platinum, and, to* a certain degree, lead decompose it with the evolution 

 of hydrogen. With bases it gives directly metallic fluorides, and 

 behaves in many respects like hydrochloric acid. There are, however, 

 several distinct individual differences, which are furthermore much 

 greater than those between hydrochloric, hydrobromic, and hydriodic 

 acids. Thus the silver compounds of the latter are insoluble in water, 

 whilst silver fluoride is .soluble. Calcium fluoride, on the contrary, is 

 insoluble in water, whilst calcium chloride, bromide, and iodide are not 

 only soluble, but attract water with great energy. Neither hydro- 

 chloric, hydrobromic, nor hydriodic acid acts on sand and glass, whilst 

 hydrofluoric acid corrodes them, forming gaseous silicon fluoride. The 

 other halogen acids only form normal salts, KC1, NaCl, with Na or K, 

 whilst hydrofluoric acid gives acid salts, for instance HKF 2 (and by 

 dissolving KF in liquid HF, KHF 2 2HF is obtained). This latter 

 property is in close connection with the fact that at the ordinary 

 temperature the vapour density of hydrofluoric acid is nearly 20, which 

 corresponds with a formula H 2 F 2 , as Mallet (1881) showed ; but a 

 depolymerisation occurs with a rise of temperature, and the density 

 approaches 10, which answers to the formula HF. 54 



The analogy between chlorine and the other two halogens, bromine 

 and iodine,, is much more perfect. Not only have their hydrates or 

 halogen acids much in common, but they themselves resemble chlorine 

 in many respects, 55 and even the properties of the corresponding 



obtained (owing to the formation of hydrofluosilicic acid). After being exposed for some 

 time, the, varnish is removed (melted) and the design drawn by the steel point is found 

 reproduced in dull lines. The drawing may be also made by the direct application of a 

 mixture of a silicofluoride and sulphuric acid, which forms hydrofluoric acid. 



w Mallet (1881) determined the density at 80 and 100, previous to which Gore 

 (1869) had determined the vapour density at 100, whilst Thorpe and Hambly (1888) 

 made fourteen determinations between 26 and 88, and showed that within this limit of 

 temperature the density gradually diminishes, just like the vapour of ace tic acid, nitrogen 

 dioxide, and others. The tendency of HP to polymerise into H 2 F 3 is probably connected 

 with the property of many fluorides of forming acid salts for example, KHF/j and 

 HaSiF 6 . We saw above that HC1 has the same property (forming, for instance, HaPtCle, 

 &o., p. 457), and hence this property of hydrofluoric acid does not stand isolated from the 

 properties of the other halogens. 



55 For instance, the experiment with Dutch metal foil (Note 16) may be made with 

 bromine just as well as with chlorine. A very instructive experiment on the direct com- 

 bination of the halogens with metals maybe made by throwing a small piece (a shaving) 

 f aluminium into a vessel containing liquid bromine ; the aluminium, being lighter, floats 

 on the bromine, and after a certain time reaction sets in accompanied by the evolution 

 of heat, light, and fumes of bromine. The incandescent piece of metal moves rapidly 



