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



ethyl-methyl ether, CH 3 — 0— C 2 H 5 , boils at 11°; but ethyl 

 alcohol, H — — C 2 H 5 , boils at 78°; in this latter substance 

 the oxygen is partly exposed by the substitution of H for 

 CH 3 . Again, in ethyl butyrate, C 2 H 5 — 0— CO— CH 2 — C 2 H 5 , 

 and in propyl propionate, C 2 H 5 — CH 2 — — CO — C 2 H 5 , it is 

 evident that the oxygen atoms occupy in each case an equally 

 central position, while the total number of atoms is the same. 

 We might therefore expect that the boiling-points of the two 

 substances would not be far apart. Experiment shows that 

 the boiling-point of the butyrate is 121°, that of the propio- 

 nate 122°. In the isomeric substance butyl acetate the oxygen 

 atoms are no longer so central, and the boiling-point rises to 

 124°*3; while in valeric acid, in which the oxygen atoms may 

 be viewed as occupying an outside place in the molecule, the 

 boiling-point rises above 180°. As another example of the 

 reduction of the total molecular attraction when the oxygen 

 occupies a central and sheltered position, we may take ether, 

 C 2 H 5 — — C 2 H 5 , boiling at 35°. If it be true that in this case 

 the attractions (A o) are greatly diminished by the distance over 

 which they are exerted, it is clear that for we may, without 

 much general effect, substitute CH 2 or some other group 

 of atoms of similar weight: this substitution produces pentane; 

 and we find that this is a body having a boiling-point differing 

 but little from that of ether. Many other examples might be 

 brought forward, were it not that their discussion would tran- 

 scend the limits of this paper. 



The use of the above hypothesis renders it difficult, at first 

 sight, to account for the formation of definite chemical com- 

 pounds. It seems that if any number of atoms of hydrogen 

 are equally attracted by one of chlorine, the combination of 

 one of them with that atom would not prevent the adherence of 

 a second and a third forming H 2 CI, H 3 CI, &c. This difficulty 

 is avoided by supposing that the chlorine atom is of such a 

 form that only one atom of hydrogen can approach sufficiently 

 closely to adhere permanently; such forms are difficult to 

 imagine, though it may be remarked that an atom in the form 

 of a ring offers in a certain sense a unique position to another 

 which instals itself inside it. The existence of molecular com- 

 pounds proves that the permanent adherence of other atoms is 

 sometimes possible, and thus affords material support to the 

 notion that the ehemica] affinity of an atom is not only exerted 

 upon those atoms with which it is combined, but upon all others 

 in its vicinity. 



