258 Mr. F. D. Brown on Molecular Attraction. 



over comparatively large distances, owing to the hydrogen of 

 one molecule shielding its companion oxygen from the approach 

 of other hydrogen. In the similarly constituted body, H 2 S, 

 the value of the molecular attraction will be 



4A 1 (/iA)+4B 1 (As)+C 1 (ss), 



in which expression A x , B l7 and Ci may be supposed to have 

 values not differing excessively from those which hold good 

 in the case of water (the sulphuretted hydrogen being supposed 

 liquid). 



We have seen that the value (s s) may possibly be small; 

 and as the attraction is exerted between atoms which cannot 

 approach each other very closely, Oi is certainly small : hence 

 this term will not probably much increase the molecular 

 attraction. The first term will remain much the same as in 

 the case of water; but the second term will have a much dimi- 

 nished value on account of the feeble affinity of hydrogen for 

 sulphur: the whole value of the molecular attraction will there- 

 fore be much smaller than in the case of water; and we should 

 expect sulphuretted hydrogen to be much more volatile than 

 water, which is actually the case. With carbon dioxide it 

 would seem, at first sight, that we have a substance which 

 should be less volatile than water ; for the heat evolved in 

 burning carbon is greater than that given out by the combus- 

 tion of hydrogen; (co) therefore is probably greater than 

 (ho); and as (c c) is greater than (00), and (00) than (hh), 

 the whole value 



4A 2 (0 0) + 4B 2 (c 0) + C 2 (c c) 



would appear to be greater than the corresponding value for 

 water. The researches of Kopp and, more recently, of Prof. 

 Thorpe have, however, shown that the atom of hydrogen 

 always occupies about the same volume, and that, if we take 

 this volume as the unit, the atom of carbon occupies a volume 

 2, while one of oxygen, when combined with only one atom 

 of carbon, occupies a volume 2*22. From these numbers we 

 learn that two contiguous molecules of C0 2 must have their 

 respective atoms of carbon more distant than the atoms of 

 oxygen in two contiguous molecules of water; for the oxygen 

 in the one case occupies a far larger volume than the hydrogen 

 in the other: hence also the important attraction (co) is exerted 

 over a much greater distance, a distance which is increased by 

 the larger volume of the atom of carbon itself. The second and 

 third terms of the above quantity are therefore greatly dimi- 

 nished, and the whole value may well be less than in the 

 case of water. 



