48 
Selected Articles. 
arseniate of soda, or the arseniate of cobalt with the oxide 
of sodium. 
Whatever may be the plan adopted to make this blue, it 
attracts moisture and loses its colour, which may be restored 
however, by subjecting it to the action of heat. A small 
quantity of iron does not prevent the appearance of a blue 
colour. 
When M. Thenard was experimenting on the blue colour 
which bears his name, he found that the union of phosphate 
of cobalt and oxide of potassium or sodium, produced a bright 
blue. The proof of the change that took place in the base 
was, that when he used too much alkali, he obtained a 
black; when he used phosphate of cobalt and alumine, de- 
composition likewise took place, and the alumine, instead of 
acting the part of an acid, as had hitherto been supposed, 
really played the part of a base. 
When oxide of cobalt is combined with glass, to obtain 
smaltz, it often happens that with the same materials, dif- 
ferent shades of colour are produced. It is of importance to 
ascertain the best plan for obtaining them of a uniform tint 
The difference is by some attributed to the presence of iron, 
but this is an error ; it is more likely that it arises from the 
heat being urged beyond the point at which the blue colour 
manifests itself, when other chemical reactions take place. 
It should be borne in mind, that the cobaltic acid is developed 
in greatest intensity under circumstances which must be 
closely studied by the operator. I believe that when the 
oxide of cobalt is pure, that the intensity of the heat some- 
times drives off a part of the oxygen and reduces a small 
portion of the base to a metallic state. 
Preparation of some blues. Take the precipitate of borate 
of cobalt, which is formed by pouring a neutral solution of 
salt of cobalt into one of borate of soda, or vice versa; wash 
the precipitate slightly and calcine it for a short time. Mix 
one part of this borate of cobalt with one or two parts of 
melted phosphate of soda, and heat the mixture to redness in 
a crucible. The borate of cobalt may be replaced by a phos- 
phate of the same base, which also affords a fine blue. The 
