1878.] 
On Residual Phenomena. 
21 
to faffts. Thus it was at one time maintained that the dis- 
tinction between animals and vegetables lay in the presence 
of nitrogen in the former and its absence in the latter. But 
as soon as improved methods of analysis showed the presence 
of nitrogen in vegetable tissues the theory was recognised as 
at variance with faffts, and was therefore abandoned. We 
may here briefly glance at certain phrases very common in 
the mouths of unscientific persons, of whatever stage of 
culture. They are apt, when encountered by an inconvenient 
fa£t, to tell us, in an off-hand manner, that there is “ no rule 
without an exception,” or sometimes even that “ the excep- 
tion proves the rule,” — a dictum quite on a par with Sir 
Thomas Browne’s celebrated “ credo quia impossibile est .” 
Now we should like any speaker or writer of this class to 
find us an exception to the law of universal gravitation, or 
to the dodtrine of definite combination just mentioned. Let 
him produce, e.g,, an element which will combine with oxygen, 
or with sulphur, or with chlorine in every conceivable pro- 
portion, and yet form not mere mixtures, but a true compound 
or true compounds. Having found such an element, let him 
further show how by its existence the “ rule ” of combination 
in definite proportion is “ proved,” or, indeed, other than 
completely refuted. Still there is a sense in which this 
saying carries with it a certain amount of truth, and which 
perhaps explains its origin. An apparent exception, if it does 
not prove, may at least confirm a rule, so soon as its nature 
is understood. Thus we know that every kind of wood, if 
dry, is combustible. If some person asserted the existence 
of an exception to this rule, and brought forward in proof, 
e.g., a piece of the fossil wood of Antigua, — i.e., silica depo- 
sited in the exaCI texture of fragments of wood which have 
been long ago decomposed, it could of course be shown that 
this substance was wood in appearance only, and that its 
incombustibility was therefore no real exception to the rule. 
Or, to take another somewhat more complicated instance : — 
Everyone who has any acquaintance with chemical and 
physical science knows what is meant by the law of iso- 
morphism ; yet to this law, when first promulgated, there 
appeared a signal exception. Arsenic and phosphoric acids 
were at once recognised as compounds mutually analogous, 
and coming within the scope of the law ; yet their soda- 
salts, as then known, did not exhibit that identity of crystal- 
line form which the law of isomorphism requires. On closer 
investigation, however, it appeared that the ordinary phos- 
phate of soda of commerce differs decidedly in its composition 
from the arseniate of soda, and, further, that there exists 
