B. Godwin— Repetition in chemical analysis. 249 
to be desired. Arsenous sulphid does not become granular 
and dense under the same circumstances. In this connection I 
may be permitted to mention that the sulphids of nickel and 
cobalt, when precipitated from boiling solutions in the manner 
recommended by Prof. Gibbs some years since, should be fil- 
tered off, and washed pete ice after precipitation. In this 
manner there is no oxydation upon the filter, even during the 
drying of the precipitate. But if the sulphids are allowed to 
stand in the solution from which they have been precipitated, 
for even a few hours, they will usually oxydize upon the filter 
during the washing. 
§ 5. 
On the wii ie the principle of es into chemical 
; by Bryant Gopw 
The method of aan so frequently and so advantageously 
employed in physical investigations, has not, so far as I am 
aware, been applied to chemical analysis. It seems at least de- 
sirable that it fog se be so applied, and : will here give hm 
ribbed filter, sa is quickly washed with water rma 
boiled to expel air, the iron is wholly in the form of ferro 
oxyd, and the process of titration may be repeated a oe time. 
After a new reduction the iron may again be determined, and 
this process may be repeated until the volume of liquid becomes 
too large to be easily handled. The following analyses were 
made to test this process : 
0°4725 gr. ammonio-ferrous sulphate required in 5 successive titra- 
tions 49°0, 47°2, 48°7, 48°0, 48°5 cubic centimeters of potassic 
hyperman nate, 1 ec. corresponding to 0°0014 gr. iron. 
e mean of these 5 determinations gives 14°31 pr. ct. iron in 
the salt. 
0°4888 gr. e- aghe in - gh g wee: titrations 49°5, 48°75, 50°5, 49°8, 
49°7, 4 ee 49°5 © ypermanganate, the mean of whic 
gives 14°23 pr. et ia 
The formula requires 1497 
Tr. 
These analyses, which 42 more practice and experience on 
my part would doubtless have corresponded much more closely, 
will at least serve to show that the principle of repeated obser- 
vations of the same quantity to be measured may sometimes be 
introduced into chemical anal 
Am. Jour. Sci.—Szconp Srries, Vou. L, No. 149.—SEpr., 1870. 
16 
