you have only to ascertain the quantity of
heat received from, and the distance of a remote body in order to know
how hot it is.[701] And the validity of this principle, known as
"Newton's Law" of cooling, was never questioned until De la Roche
pointed out, in 1812,[702] that it was approximately true only over a
low range of temperature; while five years later, Dulong and Petit
generalised experimental results into the rule, that while temperature
grows by arithmetical, radiation increases by geometrical
progression.[703] Adopting this formula, Pouillet derived from his
observations on solar heat a solar temperature of somewhere between
1,461 deg. and 1,761 deg. C. Now, the higher of these points--which is
nearly that of melting platinum--is undoubtedly surpassed at the focus of
certain burning-glasses which have been constructed of such power as
virtually to bring objects placed there within a quarter of a million of
miles of the photosphere. In the rays thus concentrated, platinum and
diamond become rapidly vaporised, notwithstanding the great loss of heat
by absorption, first in passing through the air, and again in traversing
the lens. Pouillet's maximum is then manifestly too low, since it
involves the absurdity of supposing a radiating mass capable of heating
a distant body more than it is itself heated.

Less demonstrably, but scarcely less surely, Mr. J. J. Waterston, who
attacked the problem in 1860, erred in the opposite direction. Working
up, on Newton's principle, data collected by himself in India and at
Edinburgh, he got for the "potential temperature" of the sun 12,880,000
deg. Fahr.,[704] equivalent to 7,156,000 deg. C. The phrase _potential
temperature_ (for which Violle substituted, in 1876, _effective
temperature_) was designed to express the accumulation in a single
surface, postulated for the sake of simplicity, of the radiations not
improbably received from a multitude of separate solar layers
reinforcing each other; and might thus (it was explained) be
considerably higher than the _actual_ temperature of any one stratum.

At Rome, in 1861, Father Secchi repeated Waterston's experiments, and
reaffirmed his conclusion;[705] while Soret's observations, made on the
summit of Mont Blanc in 1867,[706] furnished him with materials for a
fresh and even higher estimate of ten million degrees Centigrade.[707]
Yet from the very same data, substituting Dulong and Petit's for
Newton's law, Vicaire deduced i