12 SCIENCE PROGRESS 



magnesium and treated with acid gave very small quantities of 

 volatile hydrides, which were readily decomposed by heat. 

 Since these elements are isotopes of bismuth and lead respec- 

 tively, it was concluded that the latter two elements were also 

 capable of giving hydrides. An alloy of bismuth with mag- 

 nesium when treated with 4 n, HCl gave bismuth hydride in 

 sufficient quantity to be liquefied with liquid air and to be 

 evaporated through a heated Marsh tube. A bismuth mirror 

 was obtained. A tin magnesium alloy gave tin hydride in 

 the same way, whereas the experiment with lead gave a doubt- 

 ful result. Numerous electrical methods also failed, but by a 

 modification of Bredig's dispersion method, using a lead 

 cathode and dilute sulphuric acid as an electrolyte, the lead 

 hydride was obtained. Paneth concludes that the formation 

 of the gas takes place between the negative lead ions and the 

 positive hydrogen ions. 



It is of interest to consider the relationship between the 

 atomic number of an element and its ability to form gaseous 

 hydrides. Of the twenty elements forming gaseous hydrides, 

 all except boron possess atomic numbers which are less than 

 those of the corresponding rare gases by 1-4 units. In the 

 Staigmiiller system of classification of the elements, all of those 

 in the last four groups form gaseous hydrides. 



Nernst {Zeit. Electrochemie, 1920, 26, 323), in an experimental 

 and theoretical investigation into the properties of lithium 

 hydride, has shown that this substance is an analogue of 

 lithium chloride. The crystal form, heat of formation, atomic 

 heat, atomic volume, colouring by ultraviolet light, etc., of 

 these two substances support this analogy. This replacement 

 of hydrogen by chlorine, without any marked change in the 

 character of the compounds, resembles the change from acetic 

 to chloracetic acid. Nernst has applied the heat theorem to 

 the properties of lithium hydride and calculated the dissociation 

 of H2 into positive and negative ions according the equation 



HgEa^H' + HEaC^H') 



and finds K = 0'55 x io-2^(Co).* 



This method of dissociation may also apply to the oxygen 

 molecule, which would thus dissociate into positive and negative 

 ions. This would provide an explanation of the behaviour of 

 oxygen in autoxidation. 



Periodic Phenomena in Electrolysis. — An interesting contri- 

 bution to this subject has been made by Liebreich {Zeit. Elektro- 

 chemie, 1921, 27, 94) on the electrolysis of a solution of chromic 

 acid between iron and platinum electrodes. The periodic evolu- 

 tion of gases or the deposition and solution of oxides and metals 

 during electrolysis has long been observed, especially with iron 



