142 PRINCIPLES OF CHEMISTRY 



similar phenomenon to solution ; it is based on the capacity of metals 

 of forming unstable easily dissociating compounds 38 with hydrogen 

 similar to those which salts form with water. 



At the ordinary temperature hydrogen very feebly and rarely enters 

 into chemical reaction. The capacity of gaseous hydrogen for reaction 

 becomes evident only under a change of circumstances by compression, 

 heating, or the action of light, or at the moment of its evolution. How- 

 ever, under these circumstances it combines directly with only a very 

 few of the elements. Hydrogen combines directly with oxygen, sulphur, 

 carbon, potassium, and certain other elements, but it does not combine 

 directly with either the majority of the metals or with nitrogen, phos- 

 phorus, ifcc. Compounds of hydrogen with certain elements on which 

 it does not act directly are, however, known ; they are not obtained by 

 a direct method, but by reactions of decomposition, or of double decom- 

 position, of other hydrogen compounds. The property of Irj'drogen of 

 combining with oxygen at a red heat determines its combustibility. 

 We have already seen that hydrogen easily takes fire, and that it then 



- on their expansion, brought about by heat, and proves that metals and alloys have a 

 certain porosity.' However, Graham proved that it is only hydrogen which is capable of 

 passing through the above-named metals in this manner. Oxygen, nitrogen, ammonia, 

 and many other gases, only permeate through in extremely minute quantities. Graham 

 showed that at a red heat about 500 c.c. of hydrogen pass per minute through a surface 

 of one square metre of platinum I'l mm. thick, but that with other gasea the amount 

 transmitted is hardly perceptible. Indiarubber has the same capacity for allowing the 

 transference of hydrogen through its substance (see Chap. III.), but at the ordinary tem- 

 perature one square metre, 0'014 mm. thick, transmits only 127 c.c. of hydrogen per 

 ' minute. In the experiment on the decomposition of water by heat in porous tubes, the 

 clay tube may be exchanged for a platinum one with advantage. Graham showed that 

 by placing a platinum tube containing hydrogen under these conditions, and surrounding 

 it by a tube containing air, the transference of the hydrogen may be observed by the 

 decrease of pressure in the platinum tube. In one hour almost all the hydrogen (97 p.c.) 

 had passed from the tube, without being replaced by air. It is evident that the occlusion 

 and passage of hydrogen through metals capable of occluding it are not only intimately 

 connected together, but are dependent on the capacity of metals to form compounds of 

 various degrees of stability with hydrogen like salts with water. 



58 Palladium, as it appeared on further investigation, gives a definite compound, 

 PdoH (see further) with hydrogen ; but what was most instructive was the investigation 

 of sodium hydride, Na. 2 H, which clearly showed that the origin and properties of such 

 compounds are in entire accordance with the conceptions of dissociation. In the chapter 

 devoted to sodium we shall therefore speak more fully of this substance. 



Being a gas which is difficult to condense, hydrogen is little soluble in water ami 

 other liquids. At a hundred volumes of water dissolve 1'9 volumes of hydrogen, and 

 alcohol 6'9 volumes measured at and 760 mm. Molten iron absorbs hydrogen, but in 

 solidifying, it expels it. The solution of hydrogen by metals is to a certain degree 

 based on its affinity for metals, and must be likened to the solution of metiils in mercury 

 and to the formation of alloys. In its chemical properties hydrogen, as we shall see 

 later, has much of a metallic character. Pictet (see Note 81) even affirms that liquid 

 hydrogen has metallic properties. The metallic properties of hydrogen are also evinced 

 in the fact that it is a good conductor of heat, which is not the case with other gases 

 (Magnus). 



