318 Tlie Philosophical Society of Glasgow. 



All these ingredients are highly soluble in water, except the first and 

 the last four in the list. If, therefore, all these compounds were equally 

 insoluble, the incrustation of a boiler using the water should consist 

 almost entirely of sulphate of lime, with a small proportion of carbonate 

 of lime, and a mere trace of magnesia. Sulphate of lime is, however, 

 about a hundred times more soluble than the carbonates of lime and 

 magnesia — at least in water free from carbonic acid' — so that a con- 

 siderable quantity of the sulphate must be carried away in the water 

 discharged from time to time from the boiler. The following examples 

 may be given as illustrations of the general characters of these crusts. 

 No. 1 was from the Cunard steamer Asia, g-inch thick, and was in 

 three layers ; No. 2 was a homogeneous cake, §-inch thick, from the 

 King Orry ; No. 3, from the Cosmopolitan; No. 4, source unknown : — 



Sulphate of Lime, . 

 Carbonate of Lime, 

 Magnesia, .... 

 Water, with traces of Carbonic Acid, 24'67 

 Phosphate of Lime, Alumina, and) ..go .cq 1-34 2*40 



Oxide of Iron, . . / 



Silica, traces. traces. "57 '80 



Chloride of Sodium, . . . traces. traces. 2-04 2-16 



100-00 100-00 100-00 100-00 



These crusts differ from the insoluble matter obtained by simply eva- 

 porating sea-water in open vessels ; for that contains nearly four times as 

 much carbonate of lime as carbonate of magnesia ; while the crusts con- 

 tain a large quantity of magnesia, and little or no carbonate of lime. 

 The decomposition of soluble magnesian salts by carbonate of lime, 

 under the influence of a liquid boiling at a high temperature (say 270°), 

 is exceedingly interesting. Sulphate of magnesia and carbonate of 

 lime, boiled with water under ordinary circumstances, do not re-act 

 upon each other in the slightest degree ; but it is evident that the 

 result is brought about under pressure. The re-action with oxide of 

 manganese, which is isomorphous with magnesia, is exactly similar, and 

 is taken advantage of in the recovery of the manganese used in the 

 preparation of chlorine, as practised at the St. EoUox Chemical Works. 

 The solution of chloride of manganese is first treated with chalk or lime 

 in an open boiler, when all the iron is precipitated, but none of the 

 manganese. The clear liquor is then boiled under pressure, with a fresh 

 quantity of chalk, when the manganese is completely precipitated, 

 while the chalk is dissolved. 



Again, the condition in which the magnesia occurs is peculiar. We 

 should expect a basic carbonate ; but I find little more than a trace of 

 carbonic acid in any of the crusts. The magnesia exists essentially as 



