OUR METHOD OF DETERMINATION'. 6l 



lytical ratio, a, and is calculated to five decimal places, 

 uniformly, in this book. 



s:p = a (i) 



Now both the substance and the product are definite, well 

 known chemical compounds, as pure as it is possible for the 

 most refined chemical art to produce them. 



Hence the chemical formula of these compounds is 

 known. 



Taking our standard atomic -weights for these symbols, 

 we shall obtain the standard atomic -weight of both the sub- 

 stance and the product. 



Let us represent these known numbers by S and P, then 

 a simple division will give us the standard atomic ratio, 

 which we also calculate to five places: 



S:P = r (2) 



All the rest is done by simply comparing the analytical 

 ratios to the atomic ratio, always using the units in the fifth 

 place for this purpose. 



We shall soon learn that, as a matter of fact, the analyti- 

 cal excess 



e=za r (3) 



which is the difference between the analytical and our 

 atomic ratio, is very small, in all cases where the analytical 

 work has been done by a really good practical chemist, and 

 where the method used has been a good, well tested, method. 



In order to avoid the use of the signs minus and plus, 

 always awkward in non-mathematical books, we shall use 

 the common terms high and low to designate the character 

 of the analytical excess e. 



Namely high if it is greater, and loiv if it is less, than the 

 atomic ratio. 



Example : Mercury. 



For example, mercuric oxide Hg O yields metallic mercury, 

 Hg; both are accurately determined under the conditions 

 worked out by Erdmann and Marchand in 1844. 



The standard atomic weights are Hg = 200, O = 16, 

 exactly. Hence the reaction determines the atomic ratio r 

 as follows: Hg : Hg O = 200 : 216 = 0.92 593. 



