790 sm B. C. BRODIE OiV TlIE CALCULUS OF CHEMICAL OPEEATIONS. 



and the aspect in which every chemical substance, every portion of ponderable matter, will 

 be liore regarded is exclusively as a weight. In speaking of such weights we habitually 

 employ, by a tacit convention, the terras by which the chemical substances, of which the 

 wciglit alone is referred to, are usually designated. But this is not a strictly accurate 

 use of language; and it is necessary to observe that in the following pages, where 

 chemical substances, such as chloi'ine or alcohol or water, are mentioned, or the term 

 " a portion of matter" is employed, the objects referred to are certain weights of the sub- 

 stances under consideration, to the exclusion of all other properties. 



4. "A single weight" is a portion of ponderable matter of any specified kind con- 

 sidered as regards weight and as one object, as for example a portion of oxygen, or a 

 portion of ponderable matter consisting of oxygen and hydrochloric acid considered as 

 one object, or two portions of oxygen similarly considered. 



5. A "group of weights" is some number of single weights, such as a portion of 

 oxygen and a portion of hydrochloric acid considered as two objects, or two portions of 

 oxygen similarly considered. The single weights of which a group consists are termed 

 the " constituents" of the group, which is said to be " constituted" of them. 



6. Two portions of ponderable matter which consist of the same " weights " are said 

 to be identical as regards weight. 



It follows from this definition that what may be termed the absolute weight, or 

 Aveight in grammes, of " identical weights " is equal. 



Our knowledge as to the identical relations of ponderable matter is derived exclu- 

 sively from the science of chemistry. Were we unacquainted with the peculiar pheno- 

 mena of chemical transmutation, or were we ignorant of the circumstance that in chemical 

 change the total weight of matter is unaltered, the existence of such relations would be 

 ^ Tinknown to us. It is so important to have a clear perception of that distinction, which 

 is here made I believe for the first time, between equality and identity of weight, that 

 a few words in somewhat fuller explanation of the grounds on which this distinction 

 rests may not be deemed out of place. 



If we were to take any portion, say a gramme, of water, and observe its properties 

 on two successive days, the conditions under which the water was placed being assumed 

 as fixed, it would be found that the properties of the water were precisely the same at 

 the second as at the first observation. In this case the identity of properties would 

 be absolute, and as we know that at whatever time the observation had been made, 

 under the same conditions, the same result would have been obtained, we should 

 hence arrive at the conception of the continuous existence of one and the same object, 

 wliich we should denote with perfect precision by one and the same name, a gramme of 

 water. Now let it be assumed that some condition varies, that the temperature, for 

 example, rises from 60° to 90°, and let another observation be made of the properties of 

 the water. It would be found that these properties were no longer identical with those 

 previously observed ; that the bulk of the water had increased, and that some properties 

 had varied while others had remained constant. If the expansion of water were the 



