Green and Blue Lltramarine. 531 



chloride of barium. The sulphiii-ic acid here obtained is con- 

 tained in undecomposed ultramarine, in combination with lime 

 as gypsum, which is a never- failing admixture in clay. The 

 quantity of sulphm- liberated as sulphuretted hydrogen in the 

 decomposition of ultramarine by acid was thus estimated. 



In a flask of a litre capacity, about 0-5 grm. ultramarine was 

 brought, and the flask filled with water. About 10 drops of thin 

 starch-paste were added, and about 10 to 15 cub. ceutims. of 

 strong hydrochloric acid. The blue colour disappeared gradually, 

 and became finally milky white. By means of a burette, a solu- 

 tion of 5 grms. iodine in 1000 cub. centims. water was dropped 

 in, the mixture being continuously shaken until the charac- 

 teristic colour of the iodine appeared. The sulphuretted hydro- 

 gen could be estimated to 0*1 cub. centim., for its quantity in 

 the liquid never reached that point at which its determination 

 becomes incorrect. 



As an example of the mode of calculating the sulphuretted 

 hydrogen, the determination No V. may be given. In this 

 17*6 cub. centims. of iodine were employed, which contained 

 0-088 grm., con-esponding to 0-0117 grm., or 2-356 per cent, 

 sulphuretted hydrogen, or 2*217 per cent, sulphur; for 1 equiv. 

 sulphuretted hydrogen requires 1 equiv. iodine to decompose it 

 into hydriodic acid and sulphur. 



From the total quantity of sulphuric acid found, that portion 

 was subtracted which was contained as such in the ultramarine, 

 and the remainder was calculated as sulphur. From this the 

 quantity of sulphur was subtracted which was evolved as sul- 

 phuretted hydrogen, and the remainder gave the quantity of sul- 

 phur separated as milk of sulphur in the decomposition of ultra- 

 marine. 



The theories of the constitution of ultramarine have been as 

 various as the researches published on them. One point has 

 been agreed on, that with the exception of iron no heavy metal 

 is contained in ultramarine; but whether the product was a 

 compound of silica, alumina, soda, and sulphur, or whether the 

 iron united with sulphur was the colouring principle, has been 

 undecided. 



Eisner thinks that a small quantity of iron is essential to the 

 blue colour, but an excess injurious ; and Kressler and Priickner 

 hold a similar opinion. Guyton Morveau held that the colour- 

 ing principle was sulphide of iron. This opinion has been ex- 

 tensively received. 



Briinner, on the other hand, says that the iron plays no im- 

 portant part, for a mixture of materials free from iron gave a 

 blue, the same as that from materials containing iron. 



I have found that the iron, when present in considerable 



