l, "have been
detected in the investigation of residual phenomena." Thus, Lord
Rayleigh and Sir W. Ramsay found that nitrogen from the atmosphere was
slightly heavier than nitrogen got from chemical sources; and, seeking
the cause of this difference, discovered argon.

The Economist shows that when a country imports goods the chief means of
paying for them is to export other goods. If this were all, imports and
exports would be of equal value: yet the United Kingdom imports about
L400,000,000 annually, and exports about L300,000,000. Here, then, is a
residuary phenomenon of L100,000,000 to be accounted for. But foreign
countries owe us about L50,000,000 for the use of shipping, and
L70,000,000 as interest on the capital we have lent them, and
L15,000,000 in commissions upon business transacted for them. These sums
added together amount to L135,000,000; and that is L35,000,000 too much.
Thus another residuary phenomenon emerges; for whilst foreigners seem to
owe us L435,000,000 they only send us L400,000,000 of imports. These
L35,000,000 are accounted for by the annual investment of our capital
abroad, in return for which no immediate payment is due; and, these
being omitted, exports and imports balance. Since this was written the
figures of our foreign trade have greatly risen; but the character of
the explanation remains the same.

When, in pursuing the method of Variations, the phenomena compared do
not always correspond in their fluctuations, the irregular movements of
that phenomenon which we regard as the effect may often be explained by
treating them as residuary phenomena, and then seeking for exceptional
causes, whose temporary interference has obscured the influence of the
general cause. Thus, returning to the diagram of the Price of Tea in Sec.
4, it is clear that generally the price falls as the duty falls; but in
Mr. Denyer's more minutely wrought diagram, from which this is reduced,
it may be seen that in 1840 the price of tea rose from 3_s._ 9_d._ to
4_s._ 9_d._ without any increase of duty. This, however, is readily
explained by the Chinese War of that year, which checked the supply.
Again, from 1869 to 1889 the duty was constant, whilst the price of tea
fell as much as 8_d._ per lb.; but this residuary phenomenon is
explained by the prodigiously increased production of tea during that
period in India and Ceylon.

The above examples of the method of Residues are all quantitative; but
the method is ofte