458 



colleotive name. This word is generally only nsed when elements 

 occur in moi'e than one solid slate; that this won! should also be 

 used fbi- coniponnds is oni_y an advantage, for tiiere is no reason 

 whatever to assume an essential ditrerence for the phenomenon for 

 elements and compounds. Besides we find allotropy used already 

 several times for compounds; moreover we find it already apjilied 

 to non-solid states; thus oxygen is often called allolropic, when the 

 occuri'enee of oxygen as ozone and ordinary oxygen is referred to. 



Why then should not we generally indicate tlie occurrence of 

 different kinds of molecules by allotropy? In tins sense it was 

 already used by Prof. Smits in his theoretical considerations. Rationally 

 the occurrence of two or more solid states is then to be called jihase- 

 allotropi/, the occurrence of more kinds of molectdes ivolccuJar-dUotropy. 

 Phase allotropy will then in virtue of the above often, if not always, 

 find its gronnd in molecular allotropy 'j. 



Nothing is known of structure and size of the molecules in solid 

 state for ammonium chloride. In connection with the above this 

 sufficiently justifies the choice of the title of this paper in ni}' opinion. 



Anorganic Ckemical Laboratory of 

 the University of Amsterdam. 



Physics. — "Isothermals of di-ntomic substances and their binary 

 mixtures. XVII. Preliminary vieasurements concerning the 

 isoiherma/ of hydrogen at 20° C. from 60 to 90 atmospheres" . 

 By H. Kamerlingh OniNes, C. Dorsman and G. Holst. (Comm. 

 J 46a from the Physical Laboratory at Leiden). 



(Communicated in the meeting of June 26, 1915) 



1. Introduction. For a long time if has been the intention to 

 extend the determination of isothermals of gases at low temperatures 

 to pressures beyond the limit of 60 atmospheres, which had been 

 fixed in the first stage of the Leiden investigations. In Uommunication 

 106 (April 1908) mention was made of a first step taken towards 

 the realisation of that project. 



On the basis of the data concerning the tensile strength of glass, 

 published on that occasion, (about) fifteen manometer-tnbes had been 

 constructed, by which the divided open manometer (Comm. 44) 

 could be extended in such a manner, that the entire height of 

 mercury would correspond to a pressure of 120 atmospheres. These 



1) Smits Zeitschr. f. pbysik. Chemie. 89 257 (1915). 



