bakus] DEGREE OF CONSTANT TEMPERATURE. 83 
the data in Land, but they are not yet in sufficient number to give any 
definite hints as to the nature of the relations sought. It is necessary, 
moreover, to confine such work to data obtained from scrupulously pure 
platinum and from scrupulously pure alloys — conditions which in case 
of the data of Table 7 are not vouched for. Indeed the table gives evi- 
dence of the varied character and purity of platinum derived from dif- 
ferent sources and shows a widely different electrical behavior of nom- 
inally the same alloys. 
There is one respect, however, in which, the data of Table 7 are crucial. 
They show that extrapolations based on the equations of Avenarius and 
of Tait lead to very different high temperature results. They, therefore, 
prove that these equations are insufficient, and point out the probable 
tendency toward an anomalous thermo-electric behavior, to allotropic 
modification and polymerization even in the most stable alloys. 1 Here- 
from it follows that the pyrometric value of any alloy can only be deter- 
mined by a minute thermo-electric survey made by aid of the air-ther- 
mometer, throughout the whole range of variation of thermo-electro- 
motive force and temperature. 
Intimately connected with the present discussion are questions relat- 
ing to the relative variations of a, b } or even higher constants of the 
thermo-electric formula. Tidblom (1. c.) has endeavored to throw light 
on this subject, not without success. If a and b vanish at the same rate, 
then the data of the limit couple are as truly intrinsic, i. e., as fully de- 
pendent on the metals of the series to which the couple belongs, as any 
other. But if b vanish at a rate which is relatively very rapid as re- 
gards a, then the thermo-electric equation becomes ultimately linear. 
Extrapolations made by aid of the limit couple will therefore be more 
justifiable in proportion as the fraction _ tends toward zero. 
a 
1 Cf. Le Chatelier, loc. cit. Our own results (see Table 6) were completed iu 1884, 
but in consequence of delays in moviug the laboratory, publication was delayed (see 
Preface). 
(737) 
