TRANSACTIONS OF SECTION B. 579 



or unlike monads, such monads being indirectly combined with one another through 

 the intervening atom of oxygen. Thus five of the atoms of hydrogen in common 

 alcohol were proved to be in direct combination with the carbon, whilst the other 

 one is indirectly combined with it through the oxygen, as expressed by the formula 

 H (C.jHj). 



Another compound (called methyl-oxide) was proved to have the same em- 

 pyrical composition, but very different properties and reactions, its constitution 

 being explained by the formula H3COCH3. 



Again, two compounds of distinct reactions and properties were found to have 

 the same empirical molecular composition, C^NHg, and it was clearly proved that 

 in one of them the two atoms of carbon are directly combined thus, N 6 C H , whilst 

 in the other they are indirectly combined through the atom of nitrogen CNCH , 



An immense amount of admirable work has been done of late years (especially 

 in Germany) in working out the evidences of the atomic order of complex organic 

 bodies, and in thereby obtaining a command of their reactions. 



Evidences of the same kind have been obtained of the atomic arrano'ement of 

 some few of the simplest inorganic bodies, and it is to be hoped that ere long 

 chemists will recognise the importance of examining the constitution of salts with 

 the aid of the principles established in organic chemistry. 



The foundation is already laid by our knowledge of the constitution of such 

 compounds as 



HOH, HOK, HONH,, HONO, 

 HOPbOH, HOPbONO,, HOCOK, HOSOfI 



O 0, 



and there is a strong probability regarding the atomic constitution of many other 

 water compounds, cy. SO.H^ + H^O = OS(OH)„ SO5H, + H^O = S(OH)g. 



Amongst the extensions of our means of examining the physical properties of 

 matter, and thereby discovering new varieties of matter for chemical investigation 

 spectrum analysis has played an important part, and is no doubt destined to'do far 

 more. It has already led chemists to the discovery of several previously unknown 

 elements, and has led to the detection of various known elements in distant masses 

 of which we had previously no chemical knowledge. 



Up to this point the growth of the atomic theory will be seen, from the general 

 outline which I have endeavoured to trace, to have consisted mainly in the more 

 and more full and exact identification of each elementary atom, and in the accu- 

 mulation of more and more varied and accurate evidences of its functions in 

 relation to other atoms. A step was made towards a knowledge of the general 

 relations of atoms to one another by their preliminary classification according to 

 their best-known values. 



But a far greater step has been more recently made, one which is evidently 

 destined to lead to most important results. 



It was discovered that if we arrange the elements in the empirical order of their 

 respective atomic weights, beginning with hydrogen and proceeding thence step by 

 step to the heaviest atom, we have before us a natural series with periodically 

 recurrent changes in the chemical and pliysical functions of it.s members. 



Of course the series is imperfect, and exhibits gaps and irregularities 5 but what 

 view of natural order has been complete in its infancy ? 



Some of the gaps have already been filled up by the discovery of elements 

 possessmg the anticipated properties. The generalisation affords a brilliant addition 

 to the previous corroborations of the reality of the units of matter which chemists 

 have discovered. 



Chemists have as yet taken but little account of atomic motion; althou"-h 

 the most perfect explanation of a chemical reaction consists of a statement of the 

 atomic interchange which takes place between two molecules ; or the chano-e of 

 mutual combination between the atoms in one molecule. ° 



It has, however, been proved that the heat of combination affords a measure 

 of its force ; and we know that in giving off heat particles of matter undergo a 

 diminution of velocity of motion. We see, accordingly, that substances capabfe of 



pp 2 



