154 PROFESSOR TAIT ON A FIRST APPROXIMATION 
impression was one of disappointment, as I imagined it depended on some 
peculiarity of the platmum metals, which I had hoped would furnish me with 
the means of accurately measuring high temperatures (by a process described 
in previous notes of this series). As this hope may possibly not be realised, 
I can as yet make only rough approximations to an estimation of the tempera- 
tures of these neutral points. 
“So far as I am aware, the phenomenon discovered by Cummine and 
analysed by THomson has hitherto been described thus: When the tempera- 
ture of the cold junction is below the neutral point, the gradual raising of the 
temperature of the other produces a current which increases in intensity till 
the neutral point is reached, thenceforth diminishes ; vanishes when one junction 
is about as much above the neutral point as the other is below it, and is reversed | 
with gradual increasing intensity as the hot junction is farther heated. To 
discover how my recent observation affects this statement, I first simply heated 
one junction of a circuit of iron and (hard) platinum gradually to whiteness, 
by means of a blowpipe, and observed the indications of a galvanometer—both 
during the heating and during the subsequent cooling when the flame was 
withdrawn. The heating could obviously not be effected at all so uniformly 
as the cooling; but, making allowance for this, the effects occurred in the 
opposite order, and very nearly at the same points of the scale in the descent 
and in the ascent. [I have noticed a gradual displacement of the neutral 
points when the junction was heated and cooled several times in rapid succes- 
sion; but as my galvanometer, though it comes very quickly to rest, is not 
quite a dead-beat instrument, I shall not farther advert to this point till I have 
made experiments with an instrument of this more perfect kind, which is now 
being constructed for me.] The observed effect of heating, then, was a rise 
from zero to 110 scale divisions when the higher temperature was that of the 
first neutral point, then descent to 95 at a second neutral point, then ascent 
to a third, descent to a fourth, neither of which could be at al] accurately 
observed, and finally ascent until the junction was fused. 
« With an alloy of 15 per cent. iridium and 85 per cent. platinum, the goleaeee 
meter rose to 53°5 at a neutral point, then fell to — 50 at a second, then rose to a 
third, at — 39°5, and thence fell, but I could not observe a possible fourth 
neutral point on account of the fusion of the iron. As shown on the plate, the 
first of these occurs at about 240° C. of a mercurial thermometer. 
“With another alloy supposed to be of the same metals, but of which I 
do not yet know the composition, also made into a junction with iron, the 
behaviour was nearly the same, but the readings at the successive neutral points 
were 28, — 137, —132. The temperature of the first is about 200° C. by mer- 
curial thermometer. 
“ An iron-palladium circuit showed no neutral points within the great range: 

