494 Sir W. Ramsay and Dr. B. D. Steele on the 



at least within certain limits. But for low pressures separate 

 estimations of compressibility were made with a special 

 apparatus designed by one o£ us for the ]iurpose (Trans. 

 Chem. Soc. 1902, vol. Ixxxi. \). 117')). We had intended to 

 investigate the molecular w^eights of a number of substances 

 containing various elements ; but the results W'hich some 

 compounds of carbon and hydrogen, and of carbon, hydrogen, 

 and oxygen have led to are so remarkable that it appears 

 desirable not to delay publication. We may here anticipate 

 our conclusions, in order that the importance of each point 

 may not be overlooked. Our evidence goes to show^ that the 

 densities of certain compounds calculated for zero pressure 

 are not proportional to their molecular weights deduced from 

 the atomic w-eights of the elements which they contain. 

 This conclusion involves one^ or it may be several, of the 

 following assumptions : — 



1. The substances employed may not have been pure ; 



2. The methods of experiment may not be capable of giving 

 sufficiently accurate results ; 



3. Avogadro^s hypothesis may not hold for vapours for one 

 of the reasons below. 



(a) The vapours may adhere to the glass and increase 



their apparent density ; 

 (6) The vapours may contain some complex molecular 

 groups, or, in other w^ords, they may display partial 

 molecular association at the temperature of experi- 

 ment, even under very low^ pressure ; 

 [c) The atomic w^eights of the elements may alter their 

 values according to the ratios between the number 

 of atoms in the compounds. 

 The first two suppositions, until disproved, are of course 

 by far the most likely, and w^e shall therefore commence 

 with a description of the sources and preparation of the com- 

 pounds under experiment, in order that our readers may be 

 in a position to judge of their probable purity. 



Preparation of the Comjjounds. 



These were : hexane, two octanes, benzene, toluene, ether, 

 and methyl alcohol. The first three we ow^e to the kindness 

 of Prof. Sydney Young. Beyond drying them with phos- 

 phoric anhydride and distilling them into our weighing-bulbs 

 they underwent no process of purification. 



1. Normal hexane (Young, Trans. Chem. Soc. vol. Ixxiii. 

 pp. 910-913). — This specimen was fractionated from petro- 

 leum ether. The fractionation was carried out with a 

 combined regulated temperature still-head and six-column 



