664 EEPOBT— 1892. 



^Ve can all sympathise with the members of the Section of 1834 in their desire 

 to obtain a imifoini system of chemical notation, for at that time several very 

 different systems seem to have been in use. Although the report is a short one, it 



i)robably directed the attention of chemists to the desirability of avoiding confusion 

 3y the use of various systems, and since that period many advances have been 

 made. 



There is now little necessity for every chemist to ' state explicitly the exact 

 quantities which he intends to represent by his symbols ' for the accurate determi- 

 nations of atomic weights by many chemists — and we must not omit to mention the 

 work of Stas (whose death we have had to deplore since the last meeting of the 

 British Association) — have given us a series of numbers which are in the hands of 

 all chemists, so that, except in the cases where great refinement is requisite (or 

 when the atomic weight has not been universally accepted) there is no need to 

 state the values of the symbols. 



That great advances have been made in chemical notation is well known to all ; 

 even in my own short experience I have had to learn several different methods. 

 When I began to work at chemistry I was told that sulphate of lead was to be 

 expressed by the formula PbO,S03. Ilofmann taught me that it should be PbSO, ; 

 then Gerhardt doubled the atomic weights of oxygen and sulphur, and the formula 

 became PbjSO^ ; Oannizzaro showed that the atomic weight of lead should also be 

 doubled, and the formula again became PbSOj, but representing twice as much as 

 formerly; then Frankland taught me to write SOjPbo" as the expression of the 

 graphic formula — 



o o 



//^\ . 



o o 





which not only states that the compound contains 207 of lead, 32 of sulphur, and 

 64 of oxygen, but that the sulphur is hexad, and is combined with two atoms of 

 dyad oxygen, and witli a dyad compound radical containmg one atom of lead and 

 two of oxygen ; and of all the formulae just given this is the only one which 

 satisfies the requirements which Dalton thought necessary in 1835, namely, to 

 indicate not only the weights of the elements present, but also their arrangeujent. 

 It may be objected that we do not know that this formula really represents the 

 arrangements of the atoms in plumbic sulphate, but there can be very little doubt 

 that the four atoms of oxygen in the compound are not all in the same condition, 

 for if we examine the properties of sulphuric acid (from which the sulphate of lead 

 is derived by the replacement of the hydrogen by lead), we find that two of the 

 atoms of oxygen are more closely associated with the hydrogen than are the other 

 two, and as there is some evidence, although perhaps not verv conclusive, that 

 sulphur may be capable of combining with six monad atoms, although no such 

 compound is yet known, it does not seem unreasonable to suppose that sulphuric 

 acid is really — 



O O— H 



O 0-H 



What the nature of the attraction that holds the atoms together may be is not 

 known, but it is more probably of a character similar to that of gravity wliicli 

 holds together sun and planets "than of the nature of cohesion which would hold 

 the atoms rigidly together ; the atoms in each molecule are therefore most probably 

 in a state of rotation around, or of vibration to and from, the central atom which 

 holds them together. The pictorial representation in a plane does not therefore 

 truly express the position of the atoms, but merely the relations existing between 

 them. In organic chemistry the use of formulae expressing such a relation has 



