The most complete exhibition of the colouring or mantling of metals was attained by the late Cav. Nobili, professor of physical science in Florence. The general mode in which these colours are produced is thus explained by him:[1]—

"A point of platinum is placed vertically at the distance of about half a line above a lamina of the same metal laid horizontally at the bottom of a vessel of glass or porcelain. Into this vessel a solution of acetate of lead is poured so as to cover not only the lamina of platinum, but two or three lines of the point as well. Lastly, the point is put in communication with the negative pole of a battery, and the lamina with the positive pole. At the moment in which the circuit is completed a series of coloured rings is produced on the lamina under the point similar to those observed by Newton in lenses pressed together."

The scale of colours thus produced corresponds very nearly with that observed by Newton and others in thin plates and films, but it is fuller, for it extends to forty-four tints. The following list, as given by Nobili, is divided by him into four series to agree with those of Newton: the numbers in brackets are those of Newton's scale. The Italian terms are untranslated, because the colours in some cases present very delicate transitions.[2]

First Series.
1.Biondo argentino (4)[3].6.Fulvo acceso.
2.Biondo.7.Rosso di rame (6).
3.Biondo d'oro.8.Ocria.
4.Biondo acceso (5).9.Ocria violacea.
5.Fulvo.10.Rosso violaceo (7).
Second Series.
11.Violetto (8).20.Giallo acceso.
12.Indaco (10).21.Giallo-rancio.
13.Blu carico.22.Rancio (13).
14.Blu.23.Rancio-rossiccio.
15.Blu chiaro (11)24.Rancio-rosso.
16.Celeste.25.Rosso-rancio.
17.Celeste giallognolo.26.Lacca-rancia (14).
18.Giallo chiarissimo (12).27.Lacca.
19.Giallo.28.Lacca accesa (15).
Third Series.
29.Lacca-purpurea (16).34. Verde-giallo (20).
30.Lacca-turchiniccia (17).35. Verde-rancio.
31.Porpora-verdognola (18).36. Rancio-verde (21).
32.Verde (19).37. Rancio-roseo.
33.Verde giallognolo.38.Lacca-rosea (22).
Fourth Series.
39.Lacca-violacea (24).43.Verde-giallo rossiccio (28).
40.Violaceo-verdognolo (25). 44.Lacca-rosea (30).
41.Verde (26).
42.Verde-giallo (27).

"These tints," Professor Nobili observes, "are disposed according to the order of the thin mantlings which occasion them; the colour of the thinnest film is numbered 1; then follow in order those produced by a gradual thickening of the medium. I cannot deceive myself in this arrangement, for the thin films which produce the colours are all applied with the same electro-chemical process. The battery, the solution, the distances, &c., are always the same; the only difference is the time the effect is suffered to last. This is a mere instant for the colour of No. 1, a little longer for No. 2, and so on, increasing for the succeeding numbers. Other criterions, however, are not wanting to ascertain the place to which each tint belongs."

The scale differs from that of Newton, inasmuch as there is no blue in Nobili's first series and no green in the second: green only appears in the third and fourth series. "The first series," says the Professor, "is remarkable for the fire and metallic appearance of its tints, the second for clearness and brilliancy, the third and fourth for force and richness." The fourth, he observes, has the qualities of the third in a somewhat lesser degree, but the two greens are very nearly alike.

It is to be observed, that red and green are the principal ingredients in the third and fourth series, blue and yellow in the second and first.

[1] See "Memorie ed Osservazioni, edite et inedite del Cav. Professor Nobili," Firenze, 1834.

[2] The colours in some of the compound terms are in a manner mutually neutralising; such terms might, no doubt, be amended.