PRESENT PROBLEMS 263 



liquid air is to be purchased or cheaply made, on the combinations of 

 substances which are indifferent to each other at ordinary tempera- 

 tures. Yet the research must be a restricted one, for most substances 

 are solid at 185, and refuse to act on each other. It is probable, 

 however, that at low temperatures compounds could be formed in 

 which one of the elements would possess a greater valency than that 

 usually ascribed to it; and also that double compounds of greater 

 complexity would prove stable. Valency, indeed, appears to be in 

 many cases a function of temperature; exothermic compounds, as is 

 well known, are less stable, the higher the temperature. The sudden 

 cooling of compounds produced at a high temperature may possibly 

 result in forms being preserved which are unstable at ordinary tem- 

 peratures. Experiments have been made in the hope of obtaining 

 compounds of argon and helium by exposing various elements to the 

 influence of sparks from a powerful induction coil, keeping the walls 

 of the containing- vessel at the temperature of liquid air, in the hope 

 that any endothermic compound which might be formed would be 

 rapidly cooled, and would survive the interval of temperature at 

 which decomposition would take place naturally. But these experi- 

 ments have so far yielded only negative results. There is some indi- 

 cation, however, that such compounds are stable at 1500. It might 

 be hoped that a study of the behavior of the non-valent elements 

 would have led to some conception of the nature of valency ; but so 

 far, no results bearing on the question have transpired. The condi- 

 tion of helium in the minerals from which it is obtainable by heat is 

 not explained; and experiments in this direction have not furnished 

 any positive information. It is always doubtful whether it is advis- 

 able to publish the results of negative experiments; for it is always 

 possible that some more skilled or more fortunate investigator may 

 succeed, where one has failed. But it may be chronicled that attempts 

 to cause combination between the inactive gases and lithium, potas- 

 sium, rubidium, and caesium have yielded no positive results; nor 

 do they appear to react with fluorin. Yet conditions of experiment 

 play a leading part in causing combination, as has been well shown 

 by Moissan with the hydrides of the alkali-metals, and by Guntz, 

 with those of the metals of the alkaline earths. The proof that sodium 

 hydride possesses the formula NaH, instead of the formerly accepted 

 one, removes one difficulty in the problem of valency; and SrH 3 

 falls into its natural position among hydrides. 



A fertile field of inorganic research lies in the investigation of struc- 

 ture. While the structure of organic compounds has been elucidated 

 almost completely, that of inorganic compounds is practically un- 

 developed. Yet efforts have been made in this direction which appear 

 to point a way. The nature of the silicates has been the subject of 

 research for many years by F. W. Clarke; and the way has been 



