Mr. M. M. P. Muir on Chemical Notation. 11 



the formation of chemical equations, or in the general methods 

 of chemical research. The meaning of the words " atomic 

 theory," and the distinction between the generalization and 

 convention upon which our chemical notation is really based, 

 and the hypothesis upon which it is so often said to be based, 

 are so ably pointed out by Dr. Wright in the paper referred 

 to*, and in a previous paper, that I need do no more than 

 refer any one who wishes for clear ideas on the subject to 

 those papers. 



22. Premising, then, that chemical notation does not assume 

 the existence of "atoms," but admitting that if the atomic 

 hypothesis were granted the existing chemical notation would 

 readily adapt itself to the necessities of an atomic notation, let 

 me pass on to consider some of the reasons which appear to 

 me to favour the adoption of the molecular theory of matter. 

 " A molecule may be denned as a small mass of matter, the 

 parts of which do not part company during the excursions 

 which the molecule makes when the body to which it belongs 

 is hot." This quotation from Prof. Clerk Maxwell's 'Treatise 

 on the Theory of Heat,' gives a clear definition of what is 

 meant by the term molecule. The theory in question sup- 

 poses that any given piece of matter consists of a definite 

 number of molecules, that all the molecules of the same sub- 

 stance contain the same quantity of matter, that a molecule 

 may consist of " several distinct portions of matter held to- 

 gether by chemical bonds," and, finally, that " the molecules 

 of all bodies are in a state of continual agitation." 



23. The molecular theory of matter is to be distinguished 

 from a theory which would assert that there is an absolute 

 limit to the divisibility of matter. What the theory asserts is, 

 to use Prof. Maxwell's words, " that after we have divided a 

 body into a certain finite number of constituent parts called 

 molecules, then any further division of these molecules will de- 

 prive them of the properties which give rise to the phenomena 

 observed in the substance." 



24. The " Daltonian atomic theory " cannot of course be 

 regarded as identical with that which I have just sketched. It 

 appears to me, however, that the atomic theory of the chemist 

 is based upon and is an outcome of the molecular theory of the 

 physicist, and that we can only rightly understand the former 

 when we have gained a knowledge of the latter, — in fact that 

 the atomic theory, so called, is implied in the molecular theory 

 of matter, and stands or falls with it. 



25. One of the objections raised by Dr. Millsf against. 



* Phil. Mag. [IV.] vol. xliii. p. 503. 

 t Laboratory, 18G7, p. 6. 



