CEROMATES OF POTASH 797 



1871 1872 

 Ibs. value Ibs. value 



From France . . 56,426 5,613 56,426 5,613 



other Countries . 22,075 1,621 22,075 1,621' 



Total . . 78,501 7,234 78,501 7,234 



CHOKE DAMP, After Damp, Black Damp. Terms employed by miners for 

 the atmosphere of coal-mines, after an explosion of fire-damp, which is usually air 

 deprived of its oxygen and largely mixed -with carbonic acid. Choke damp is also used 

 to express the atmosphere of carbonic acid, which accumulates in deserted shafts, in 

 abandoned workings, in wells, or in brewers' vats. It is removed by throwing lime- 

 water into the space it occupies, and agitating the air. 



CHROMO LITHOGRAPHY. Printing in colours from lithographic stones. 

 See LITHOGRAPHY. 



CHROMASCOPE. An instrument for showing colours. 



CHROMATES. Saline compounds of chromic acid with bases ; or, chromic acid 

 in which the hydrogen is replaced by a metal. See CHROMIUM and CHROMIC ACID. 



CHROMATES OF XiEAD. Some of these compounds occur native, forming 

 the minerals known as Crocoisite and Melanochroite. The chromates of lead are also 

 prepared artificially, and largely used as pigments, the neutral chromate forming 

 Chrome Yellow, and the basic chromate Chrome Red. The chrome yellow of the painter 

 is a rich pigment of various shades, from deep orange to the palest canary yellow. It 

 is made by adding a limpid solution of the neutral chromate of potash, to a solution 

 equally limpid of acetate or nitrate of lead. A precipitate falls, which must be well 

 washed and carefully dried out of the reach of any sulphuretted vapours. A lighter 

 shade of yellow is obtained by mixing some solution of alum or sulphuric acid with 

 the chromate before pouring it into the solution of lead ; and an orange tint is to be 

 procured by the addition of subacetate of lead in any desired proportions. 



It was ascertained by MM. Kiot and Delisse, that the proportion of chromic acid in 

 chromate of lead may be much diminished without any injury to the colour, and that 

 the same colour is produced with 25 parts of neutral chromate for 100 of chrome yellow, 

 as when 54 parts are used. They give the following formula for the preparation of 

 this pigment : Acetate of lead is dissolved in water, and sulphuric acid in quantity 

 necessary to convert the oxide of lead into sulphate is added. The clear liquid contains 

 acetic acid and may be drawn off", and preserved for the preparation of fresh acetate of 

 lead. The sulphate of lead is washed and treated with a hot solution of neutral 

 chromate of potash, 25 parts being used for every 75 parts of sulphate of lead. The 

 liquid then contains sulphate of potash which may be made available, and the precipi- 

 tate consists of chromate of lead. 



Chrome yellow may be prepared according to Liebig, by digesting sulphate of lead 

 in a warm solution of chromate of potash. It may also be obtained by digesting 

 100 parts of freshly-precipitated chloride of lead with 47 parts of bichromate of potash. 

 This is Anthon's process. 



To prepare chrome red, Kunge directs an intimate mixture to be made of 448 Ibs. 

 of litharge, 60 Ibs. of common salt, and 500 Ibs. of water. As soon as the mass 

 becomes white and swells up considerably, more water is added to prevent it from 

 becoming too hard. After four or five days, the mass becomes a compound of chloride 

 and hydrated oxide of lead. Without separating the mother-liquor, which contains 

 undecomposed chloride of sodium and soda, 150 Ibs. of powdered bichromate of potash 

 are to be added, and the whole well stirred together, and finally washed. 



Liebig and Wohler contrived a process for producing a subchromate of lead of a 

 beautiful vermilion hue. Into saltpetre, brought to fusion in a crucible at a gentle 

 heat, pure chrome yellow is to be thrown by small portions at a time. A strong 

 ebullition takes place at each addition, and the mass becomes black, and continues 

 BO while it is hot. The chrome yellow is to be added till little of the saltpetre 

 remains undecomposed, care being taken not to overheat the crucible, lest the colour 

 of the mixture should become brown. Having allowed it to settle for a few minutes, 

 during which the dense basic salt falls to the bottom, the fluid part, consisting of 

 chromate of potash and saltpetre, is to be poured off, and it can be employed again in 

 preparing chrome yellow. The mass remaining in the crucible is to be washed with 

 water, and the chrome red being separated from the other matters, it is to be dried 

 after proper edulcoration. It is essential for the beauty of the colour, that the saline 

 solution should not stand long over the red powder, because the colour is thus apt to 

 become of a dull orange hue. The fine crystalline powder subsides so quickly to the 

 bottom after every ablution, that the above precaution may be easily observed. 



CHROMATES OP POTASH. Three of these salts are known, but only two 

 of them are used in the arts, namely, the neutral chromate of potash KO.CrO 1 

 (X'CrO 4 ), and the acid or bichromate of potash, K0.2Cr0 3 (X'Cr'O 7 ). 



