370 THE CHEMICAL PROBLEMS OF TO-DAY. 



Nevertheless something has been achieved, since an increased interest 

 lias been drawn towards pyro-chemical research. 



To-day new methods of experiment permit of a comparatively easy 

 determination of the vapor density and consequently of the molecular 

 state of the substances at the highest temperatures. 



Numerous inorganic compounds, above all the very elements, have 

 been studied in regard to their vapor density at a white heat. 



While many of them, as oxygen, nitrogen, sulphur, and mercury, re- 

 main unchanged under such conditions, the molecules of chlorine, bro- 

 mine, and iodine, respectiv^ely, were split into two atoms, in conformity 

 with Avogadro's surmise of the compound nature of elementary mole- 

 cules. 



In the same manner, the vapor density, and hence the molecular 

 condition of the less volatile substances, zinc, thallium, antimony', and 

 bismuth, was sue essfully determined at a white heat. 



Careful research resulted in the exposure of the old fallacy of the 

 existence of a sulphur molecule containing six atoms. 



But how many of the problems which crowd around us at this point 

 are for the time being entirely beyond the reach of the experimenter ! 



To-day pyro-chemical work is limited to a temperature of 1700° C, 

 because vessels of porcelain and platinum, to the use of which we are 

 limited, fuse above that temperature. The possibility of performing 

 quantitative experiments at these temperatures seemed to us some years 

 ago to be an unexpected progress, but to-day we complain that the 

 trivial cause of a want of proper vessels forbids us to increase the tem- 

 perature up to 2000° or 3000° C. There is no doubt that we should 

 arrive at new unthoughtof facts, that the splitting of still other ele- 

 mentary molecules would be possible, that a new chemistry would be 

 revealed to us, if — being provided with vessels of infusible material, we 

 could work at temperatures at which water vapor could not exist and 

 at which detonating gas would be a non-inflammable mixture! 



Let us now enter other fields of physical chemistry. Golden fruit, 

 daily increasing, has been harvested upon this field during these latter 

 days. Again we see van't Hoff take the lead. His keen eye has en- 

 abled as to penetrate the nature of solution, which forms the beginning 

 of a new epoch in molecular physics. The quintessence of his discover- 

 ies may be thus expressed: 



'' Solutions of different substances in the same liquid, which contain 

 in the same volume an equal number of molecules of the dissolved sub- 

 stance, show the same osmotic pressure^ the same vapor pressure, and the 

 same freezing point. ^^ 



This surprising generalization offers the possibility of determining 

 the true molecular weight of substances by experimenting upon them in 

 solution, while heretofore this has only been possible by transforming 

 them into the gaseous state, hence only for volatile substances, since 



