54 Mr. R. Phillips on certain Substances which  [Juny, 
the properties of acids, whose only claim cousists in their 
power of combination with other bodies, and which power will, 
consistently with Berthollet’s observation, equally entitle them 
to be ranked in the very opposite class of alkalies. 
It will be readily granted that silica does not possess any one 
of the move obvious properties which characterize acids ; it is 
inodorous, tasteless, insoluble in water, or alcohol, does not affect. 
vegetable colours, and has no immediate action upon any alkali, 
earth, or metal, so as to neutralize, dissolve, or form crystalline 
compounds with them. On the other hand, there are cases in 
which it appears to act as an alkali; thus, in a finely divided 
state, silica is dissolved by the acids generally, and with the 
fluoric acid it forms a peculiar compound : it is certainly con- 
sidered that the silicated fluoric acid is a compound acid, but 
it is to be remembered that the fluoric acid possesses acidity 
without being combined with silica; and moreover, when sili- 
cated fluoric acid is mixed with water, the silica is precipitated ; 
but as this is perfectly analogous to what happens when muriate 
of antimony is poured into water, I think that analogies are 
more favourable to the alkaline, than to the acid properties of 
silica. 
With respect to alumina, it cannot for a moment be questioned 
. that its powers as a base are much more strongly marked than 
those of silica; it readily combines, when minutely divided, 
with almost every acid; and the formation of alum must be 
deemed satisfactory evidence of its saturating power with re- 
spect to acids. 
Alumina, however, resembles silica in its property of com- 
bining with the alkalies, potash and soda; and it is not, I 
believe, generally known, that with potash it so far performs 
the function of an acid, as to form a crystalline compound. 1 
have, however, procured it in crystals of considerable size, and 
they appeared to be efflorescent, but 1 have not yet subjected 
them to analysis; and as I am not aware that any crystalline 
compound of silica and potash has been formed, it must, | think, 
be admitted, that the acid, as well as the alkaline functions of 
alumina, are better defined than those of silica. 
Oxide of lead is a substance which possesses the power of. 
combining with acids and alkalies in a still more remarkable 
degree than alumina. When this oxide is dissolved in acetic 
acid, it is weil known that a certain quantity saturates the acid 
sufficiently to prevent its action upon vegetable colours, and by 
evaporation we procure sugar of lead; but if this solution be 
boiled with an additional quantity of oxide of lead, we obtain 
a compound (Goulard’s extract of lead) which is remarkable on. 
two accounts. First, it is a real subsalt, and soluble in water, 
and there :s not, that I know of, a similar instance in record. 
Secondly, the oxide of lead in excess acts so completely as an: 
