SIR B. C. BEODIE ON THE CALCULUS OF CHEMICAL OPERATIONS. 789 



Section I.— DEFINITIONS. 



It is essential for the purposes of exact reasoning that the terms employed should be 

 accurately defined. This can only be effected either by the invention of new words or 

 by the selection from the many shades of meaning which may be attached to existing 

 words of some one definite and appropriate signification, in which it shall be agreed to 

 employ them. I shall adopt the latter and more obvious course. 



1. The term "ponderable matter," in its chemical application, is a term by which 

 that class of objects is denoted, the transformations of which form the special study of 

 the chemist. Not that the property of weight can exist apart from those other proper- 

 ties of form, colour, and the like, with which, so far as our experience extends, it is 

 invariably associated, but that in the actual phase of the science this property is chiefly 

 considered, it being the only property of matter in regard to the chemical changes of 

 which we possess any exact knowledge. No further explanation of this term can 

 advantageously be offered, except by exhibiting or enumerating the objects (such as 

 water, silica, oxygen) which are comprehended in the class. 



2. A " chemical substance" is a portion of ponderable matter of which every part has 

 the same properties. 



It is often difficult to decide whether this condition be satisfied or not, but the pro- 

 priety of the above definition will be manifest from the line of argument which fs 

 applicable to such cases. Take, for example, the case of atmospheric air. To a super- 

 ficial observation, every part, however minute, of any given portion of the atmosphere has 

 the same properties. But a more exact scrutiny leads us to infer that this is really not 

 the case, but that it consists of parts, which diffuse with different ■velocities and are 

 unequally soluble in water. On these principles it has been established that the gas 

 procured from the decomposition of acetic acid is truly, in the sense of the above defini- 

 tion, one chemical substance, marsh-gas, and not two chemical substances, hydrogen and 

 methyl. 



3. "A weight" is a portion of ponderable matter of any specified kind considered as 

 regards weight. 



This application of the term a weight is only a slight extension of its ordinary use. 

 A gramme of platinum which serves for the purposes of weighing is termed a weight, 

 this being the only property of that portion of matter of which it is necessary to take 

 cognizance*. Now the weight of matter, from a different and far wider point of view, 

 in the laws of its composition and resolution, is the special subject of this investigation ; 



* The term weight is, in ordinary language, used with two distinct meanings. (1) In what may bo termed 

 its abstract signification, as denoting a certain measurable property of matter, as when wo inquire, " Whai ia 

 the weight of that loaf?" (2) In its concrete sense, as denoting certain objects, which we discriminate from 

 others by naming them from their most essential property, as when we say " Bring mo that box of weights." 

 It is in an extension of this concrete meaning that the word is here employed. It is not, of course, intended 

 to assert that the transformation of wehjhts is the only subject of chemical science, but simply to fix the atten- 

 tion upon that subject as the only topic which is here discussed. 



MDCCCXLVJ. 5 P 



