SIE B. C. BEODIE ON THE CALCULUS OF CHEMICAL OPEEATIONS. 793 



10. It is necessary to select a "unit of ponderable matter" which may serve as the 

 common measure of those chemical properties which it is our desire to investigate. 

 Such a standard, for example, would be supplied to us were we to select as the common 

 term of comparison that portion of ponderable matter of which the absolute weight is 

 one gramme. This plan would have the great advantage of proceeding upon accurate 

 and certain data, but, on the other hand, the conclusions to which it leads would be of 

 comparatively little interest. The present method aims at effecting a comparison of the 

 chemical properties of those portions of ponderable matter which, in the condition of 

 perfect gases, and compared at the same temperature and pressure, occupy equal volumes. 



In the numbers which express the specific gravity of gases a similar comparison is 

 made of the absolute and relative weights of the same portions of matter ; and it will be 

 found convenient to refer physical and chemical properties to the same standard. I 

 shall therefore define the " chemical unit of ponderable matter" as that portion of pon- 

 derable matter Avhich occupies the volume of 1000 cub. centims. at 0° C. and a pressure 

 of 7C0 millims. of mercury. The weights of the chemical unit of ponderable matter 

 may be expressed in two ways according to the object in view; as the absolute weight 

 in grammes, and as the relative weight in reference to the weight of some one unit 

 assumed as the standard of comparison. For this purpose the weight of the unit of 

 hydrogen will be selected. 



11. The volume of 1000 cub. centims. is here termed "the unit of space." 



The system of chemical measurement which has grown up around the atomic theoiy 

 is of a singular and artificial character. The chemist is accustomed to assume as his 

 standard of comparison, not some real and existing object, but a " molecule," an 

 ifnaginary arrangement of imaginary atoms, of which no precise definition, by which it 

 can be recognized, has ever been given. It should at least be shown that a system 

 thus constructed offers special facilities for thought. But in truth it has been found 

 so perplexing in practice, that the most skilled teachers* have been forced to admit 

 that the student requires to be initiated into this world of hypothesis by means of more 

 concrete and exact ideas. 



12. The term " distribution of weight" may be defined as that operation by which a 

 compound weight is resolved into its component weights, or by which it is made up 

 from those weights, regard being had to some special system of such events which is 

 the subject of consideration. 



* Two eminent chemists have recently given independent testimony to the value of a more real standard 

 than is afforded by this imaginary " molecule." Dr. Horsi.vNif, in his ' Modern Chemistry,' has adopted the term 

 " crith" to denote such a real unit, — a crith being the weight of 1000 cub. centims. of hydrogen at standard tem- 

 perature and pressure. Professor Williamson has, from similar motives of utility, adopted an " absolute volume " 

 of 11-2 litres, which is the bulk of a gramme of hydrogen, also at standard temperature and pressure. I now 

 propose to advance a»other step in the same direction, and to substitute the real for the fictitious unit, not for a 

 special object alone, or to pave the way for more important theories, but for aU the purposes of chemistry. 



See 'Modem Chemistry,' by A. W. Hofmann, 1865, pp. 121 and 130; and 'Chemistry for Student.",' by 

 A. W. Williamson, p. 4, 



