st expensive, some practical hints may be met with. We have
often wondered with what blue their deep-toned cool greens were made, as
in the landscapes of Gaspar Poussin. It was probably Cennino's _azzuro
della magna_ (German blue or cobalt.) Prussian blue is of recent
invention. We believe Mr Field considers it a good colour. It is made of
so many hues that it is difficult to procure good, and it is said to be
affected by iron. We have heard indigo complained of as a fugitive
colour; Cennino mentions it for skies with a tempera of glue. He
mentions, likewise, a green cobalt, or _azzuro della magna_. White lead,
according to him, may be used with all temperas. He says it is the only
white that can be used in pictures; the whites in the old pictures are
very pure, so that we may be satisfied of its durability. Many artists
have doubted if the white of the best painters was white lead, and many
substitutes have been proposed. We may rest assured, by the authority of
Cennino, that the fault is not in the lead, but in the vehicle, whenever
it changes. There is a letter of Titian's, in which he laments the death
of the maker of his white; it was made, therefore, we are to suppose,
with particular care, as the principal pigment for light.

Orpiment, which was so much in use in Sir Joshua's time, the ill effects
of which is visible in the President's "Holy Family" in our National
Gallery, was no great favourite in the olden time. In the note upon this
pigment, the translator takes occasion to speak of powdered glass, in
reference to a remark of Dr Ure, that powdered glass is mixed with it,
which renders it lighter. Mrs Merrifield infers from this, that it,
powdered glass, is opaque. Undoubtedly it is so in its dry state, and
probably with the glue tempera, which alone, according to Cennino, is
its proper vehicle--but mixed with oil it is transparent--and mixed in
much body with pigments, will give them great richness, and that degree
of transparency, even to pigments rather opaque, which we observe in the
substance of the pigments of the best time. China clay, and magnesia
too, are opaque in their powdered and dry state, but mixed with the
pigments, vary their power _ad libitum_, precisely by the transparency
they afford. These two latter substances have likewise a corrective
quality upon oils, and we are assured by Mr Coathupe, and have certainly
found it to be so, that magnesia is a dryer. We have boiled magnesia and
oil toget