202 HYDROGEN. 



viously distilled. It sometimes has a little zinc or iron suspended 

 or dissolved in it. 



3. THEORY OF THE PROCESS. 



The acid is not altered, but the water is decomposed ; its oxygen 

 passing to the iron, converts it into an oxide, and its hydrogen is 

 evolved ; the acid unites with the oxide of iron, and forms sulphate 

 of iron, which appears in green crystals, as soon as the mixture is 

 cold. How the acid operates to favor the decomposition is not al- 

 together clear. * 



4. PHYSICAL PROPERTIES. 



(a.) It is colorless and transparent. As commonly obtained, it 

 has a smell slightly fetid. If obtained over mercury, the odor is 

 much diminished. It is scarcely absorbed by water, unless it has 

 been freed from common air, when 100 cubic inches of that fluid 

 take up 1 J inches of the gas ; with strong pressure the water absorbs 

 one third of its volume. 



(6.) It refracts light more powerfully than any gas, agreeably to 

 the general law with respect to inflammable bodies; ratio 6.6 air 

 being l.f 



(c.) Specific gravity 0.694, air being 1, just 16 times lighter than 

 oxygen ; weight 2.116 grs. for 100 cub. in. at the medium tempera- 

 ture and pressure. f One cubic inch weighs but little more than j\ 

 of a grain, and fifty cubic inches but little more than one grain ; 

 it is the lightest form of matter hitherto obtained. " It is about 

 200,000 times lighter than mercury, and 300,000 times lighter than 

 platina." Hare. 



* This used to be called a case of disposing affinity ; the acid being disposed to 

 unite with the oxide of iron about to be formed, by the transfer of the oxygen of the 

 water to the iron ; this explanation appears to be no more than verbal, as the oxide 

 of iron cannot exert an attraction before it is in existence ; but if, as suggested by 

 Murray, the acid be supposed to exert, simultaneously, an attraction, both for the 

 oxygen of the water, and for the iron, it may thus aid the combination of the former 

 with the latter, and then the acid will combine with the oxide of iron. But there 

 is no evidence, except that which is afforded by the fact in question, that such an 

 attraction exists between the acid and the oxygen, and the acid and the iron. It ap- 

 pears to me better to say that we do not understand it, and to wait till we do, be- 

 fore we attempt to explain the fact. The heat generated by the action of the acid 

 and water, will not explain the decomposition, for the cold diluted acid will rapidly 

 evolve hydrogen gas from iron; it grows hot, it is true, during the action, but the 

 heat is not the cause, it is the effect of the action. There is another theoretical diffi- 

 culty in this experiment. The rapid evolution of gas, and especially of one whose 

 capacity for heat exceeds that of all known bodies, ought not, upon the received 

 theory of heat, to evolve that power; the mixture ought to grow cold. Again, the 

 crystallization of the sulphate of iron is rapid, and begins even before the mixture 

 is cold, and proceeds the more rapidly the colder the liquor grows; but the evolu- 

 tion of a solid from fluids ought to produce heat. 



I Henry, vol. l.p. 154. I Thomson. 



