babus.1 PYROELECTRIC PROPERTIES OF ALLOYS. 151 
curvilinear relations in question. Large differences between o^ 100 and 
a 356 occur in case of one aluminium alloy, two cobalt alloys, and one 
silver alloy; but the exceptional data of the aluminium alloy, as well as 
the cobalt alloys, have already been adverted to above in discussing the 
density and resistance data. In the case of the aluminium alloy, I sus- 
pect that some error of measurement has eluded me, whereas the two 
cobalt alloys seem to be deficient in homogeneity, as is also the silver 
alloy. These inferences are permissible, because the remaining alu- 
minium, cobalt, and silver alloys behave normally, and I am therefore 
warranted in excluding the three unmistakably exceptional data (A!, 
Co, Co) from the present considerations altogether. 
Without any essential restrictions therefore I need only fix attention 
upon the large biack dots of the chart, and from these 52 data it appears 
clearly that the alloys of platinum may be regarded as a class of mate- 
rials possessing certain generic physical properties, inasmuch as the 
effect of alloying platinum with small amounts (less than 10 per cent.) 
of any other metal is a variation of the limiting ratio of resistance and 
temperature, when the latter approaches zero, in a way that is inde- 
pendent of the special ingredients of the alloy from which data may be 
obtained. Such variation depends only on the resistance-position of this 
alloy in the class. 
In other words, if I put s t =/(£), where/ is a series of powers of t then 
s =/(0) and a —f (0):/(0), and therefore s and a, considered theoret- 
ically, have to each other relations expressible by a first differential co- 
efficient. According to the experimental result just stated, furthermore, 
/'(°) :/(0) is such a function of/(0) that the dependence of/'(0) :/(0) on 
/(0) is independent of the ingredients of the alloy by which the varia- 
tions of/(0) may be produced, provided, of course, the point of view be 
that of obtaining a broad class distinction for the platinum alloys as a 
whole, and not to discern rigorously the characteristics of the indi- 
vidual alloy. To return again to the figure with a view of examining 
the discrepancies critically, I find that the divergence of data from the 
mean curve, drawn as carefully as possible through them, is largest to- 
ward the right-hand half of the figure. This, however, is easily ac- 
counted for, since in proportion as the resistance of the alloy is greater 
the results are more and more seriously distorted by an insufficiently 
homogeneous mixture or by imperfect alloying of the ingredients of the 
metal. The alloys of nickel, perhaps, are conspicuous as occupying a 
position above the mean curve, the alloys of copper as falling below it, 
but for the other alloys a uniformly exceptional position can not be said 
to be discernible. If the alloying be imperfect the corresponding a will 
be erroneously large, and a tendency toward over-large values of a is 
the general character of the discrepancies which the figure presents. 
Probably, too, the extreme end of the curve already partakes of the 
divergence, in virtue of which, in a retrograde movement, the diagram 
position of the metal alloyed to platinum must ultimately be reached. 
(805) 
