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



at any rate be valuable in so far as tbe attention of others 

 will be drawn to the possibility of the task ; though the struc- 

 ture erected be soon thrown down, it will have set an example 

 which may lead to other and more durable buildings. 



Animated by these ideas, I venture to bring forward a small 

 attempt to link together some of the properties of chemical 

 substances, and to bring them under the general theories 

 which we owe to Dalton, to Avogadro, and to Kekule. All 

 chemical hypotheses founded on the atomic theory must ulti- 

 mately refer either to the form and structure of the atoms 

 and molecules, or to the movements executed and the forces 

 exerted by them. It is with the latter that we are now more 

 immediately concerned ; but we must first define exactly our 

 notions of the former. 



We all recognize that chemical substances are composed of 

 molecules, and that these, in their turn, are built up of atoms; 

 but if we step beyond this general statement, the same una- 

 nimity no longer prevails. With regard to the structure of 

 the molecules, there seems to be an unwillingness to believe 

 that the atoms are really arranged in the molecule in the 

 manner indicated by our ordinary formulae; yet it seems im- 

 possible to form any other conception concerning them than 

 this purely concrete one. If an atom is combined directly 

 with another, it must surely be contiguous to that other ; and 

 if several atoms are combined with one, they must, I imagine, 

 be ranged round that one. Again, if a group of atoms passes 

 unaltered through several reactions while the other portions of 

 the molecule are replaced or broken up, it seems but reason- 

 able to suppose that the atoms forming such a group are 

 actually placed together in space. When, further, we learn 

 from the researches of Captain Abney and of Dr. Russell 

 that the existence of such groups of atoms is registered in the 

 absorption-spectra of the compounds, that we can, as it were, 

 see them in the molecule, we shall not take up an unsafe posi- 

 tion if we assume that the ordinary formulas indicate real place 

 in the molecule. While making this assumption, I do not 

 wish for a moment to imply that the atoms are arranged in 

 that particular form which we are accustomed to give to our 

 graphic formulae, but merely to assert that the connecting- 

 link of the formula indicates juxtaposition of the correspond- 

 ing atoms.. 



It is now known with certainty that the molecular volumes 

 of substances are . but slightly altered by combination — that 

 is to say, that under comparable conditions an atom of 

 any substance generally occupies about the same space, with 

 whatever atoms, similar or dissimilar, it may be combined. 



