262 INORGANIC CHEMISTRY 



lative, rather than based on experiment. The existence of many 

 peroxidized substances, such as percarbonates, perborates, persul- 

 phates,and of crystalline compounds of salts with hydrogen peroxide, 

 makes it difficult to draw any indisputable conclusions as regards 

 valency from a consideration of oxygen compounds. Moissan's bril- 

 liant work on fluorides, however, has shown that SF 6 is capable 

 of stable existence, and this forms a strong argument in support of 

 the hexad character of sulphur. The tetravalency of oxygen, under 

 befitting conditions, too, is being acknowledged, and this may be 

 reconciled with the existence of water of crystallization, as well as of 

 the per-salts already mentioned. The adherence of ammonia to many 

 chlorides, nitrates, etc., points to the connecting link being ascribable 

 to the pentavalency of nitrogen ; and it might be worth while investi- 

 gating similar compounds with phosphoretted and arseniuretted 

 hydrogen, especially at low temperatures. 



The progress of chemical discovery, indeed, is closely connected 

 with the invention of new methods of research, or the submitting of 

 matter to new conditions. While Moissan led the way by elaborating 

 the electric furnace, and thus obtained a potent agent in temperatures 

 formerly unattainable, Spring has tried the effect of enormous pres- 

 sure, and has recently found chemical action between cuprous oxide 

 and sulphur at ordinary temperature, provided the pressure be raised 

 to 8000 atmospheres. Increase of pressure appears to lower the tem- 

 perature of reaction. It has been known for long that explosions will 

 not propagate in rarefied gases, and that they become more violent 

 when the reacting gases are compressed: but we are met with diffi- 

 culties, such as the non-combination of hydrogen and nitrogen, even 

 at high temperature and great pressure ; yet it is possible to measure 

 the electromotive force (0.59 volt) in a couple consisting of gaseous 

 nitrogen and gaseous hydrogen, the electrolyte being a solution of 

 ammonium nitrate saturated with ammonia. Chemical action be- 

 tween dissolved hydrogen and nitrogen undoubtedly occurs; but it is 

 not continuous. Again we may ask, Why? The heat evolution should 

 be great; the gain of entropy should also be high were direct combina- 

 tion to occur. Why does it not occur to any measurable extent? Is 

 it because for the initial stages of any chemical reaction, the reacting 

 molecules must be already dissociated, and those of nitrogen are not? 

 Is that in any way connected with the abnormally low density of gas- 

 eous nitrogen? Or is it that, in order that combination shall occur, 

 the atoms must fit each other; and that, in order that nitrogen and 

 hydrogen atoms may fit, they must be greatly distorted? But these 

 are speculative questions, and it is not obvious how experiments can 

 be devised to answer them. 



Many compounds are stable at low temperatures which dissociate 

 when temperature is raised. Experiments are being made, now that 



