comprise _Cerulian Blue_ or _Coeruleum_, _Cobalt Blue_, _Smalt_, _Royal Blue_, _Dumont's Blue_, _Saxon Blue_, _Thénard's Blue_, _Leithner's Blue_, _Hungary Blue_, _Dutch Ultramarine_, _Zaffre or Enamel Blue_, _Vienna Blue_, _Paris Blue_, _Azure_, &c., and are obtained by the action of heat on mixtures of earthy or metallic bases with cobalt. They are divisible into three classes--the stannic cerulian blue, the aluminous cobalt blues, and the siliceous smalts. Of these, the first possesses the least depth; the second hold a middle position; while the third are marked by exceeding richness. Although not to be ranked with ultramarine, the stannic and aluminous blues may be described as durable, or at least as durable rather than semi-stable. There are, as we have before observed, different degrees of permanence, and the blues in question are not readily affected. With regard to smalts, they are, as artist's pigments, inferior in stability to other blues of cobalt. 123. CERULIAN BLUE, or _Coeruleum_. Under the name Coëlin there has of late years been imported from Germany the cobalt blue with a tin base to which reference has just been made. This comparatively new pigment--which likewise contains or is mixed with gypsum, silica, and sometimes magnesia--has the distinctive property of appearing a pure blue by artificial light, tending neither to green on the one hand nor to purple on the other. This advantage, added to its permanence, has conferred a popularity upon coeruleum which its mere colour would scarcely have gained for it. A light and pleasing blue, with a greenish-grey cast by day, it possesses little depth or richness, and is far excelled in beauty by a good aluminous cobalt. A certain chalkiness, moreover, somewhat detracts from its transparency, and militates against its use in water. It is in oil, and as a night colour, that coeruleum becomes of service, as our present system of lighting picture galleries by gas affects the purity of blues generally. If those galleries were illuminated by means of the electric light, we have it on the authority of Chevreul that all colours and shades would show as well as by day: the same purpose would be answered by the magnesium light. Some artificial lights are the ruin of colours; in the soda flame (alcohol and salt) for instance, yellow chromate of lead appears white, while red ochre and aniline blue appear black.