MECHANICS AND USEFUL AKTS. 53 



granular. The thickness of the whole crust was about three-eighth of an 

 inch : 



Sulphate of lime SI. 6 



Magnesia 4.2 



Silica 2.8 



Peroxyd of iron 2.4 



Salt 0.7 



"\Vater of crystallization 7.7 



Carbonic acid 0.6 



100. 



The next analysis was that of scale taken from the same boiler, which was 

 worked for the same length of time, as in the former experiment, but care 

 was taken to " blow off " regularly. The scale in this case was only one- 

 sixteenth of an inch thick only one-sixth the thickness of that formed 

 when " blowing off" was neglected: 



Sulphate of lime 94.5 



Magnesia 1.5 



Peroxyd of iron 0.5 



Salt.." 1.1 



Water . . 2.4 



100. 



These analyses show that the sulphate of lime is the main ingredient of 

 the scale deposited by sea water. They also afford very satisfactory evidence 

 regarding the way to prevent incrustations by care in blowing off the satu- 

 rated water regularly. 



The following is the method proposed by Mr. Xapier for the prevention of 

 incrustations in all boilers. He analyzes the water to be used, and if found 

 to contain only the bi-carbonate of lime in suspension, there is no difficulty 

 in preventing it from forming scale. The carbonate of lirne separates from 

 the water at a high heat, and is kept suspended in the boiler while the water 

 is hot; but when the boiler is stopped, it falls to the bottom in cooling, and 

 and when cold it hardens, adheres to the metal, and forms a crust. A boiler 

 using hard fresh water containing carbonate of lime has thus a thin layer of 

 scale formed every night, and at last it accumulates to a thick stony crust, 

 which almost prevents the passage of the heat from the fire to the water. To 

 prevent such scale, the plan to be adopted is simple. In about an hour after 

 the engine is stopped every evening, and when the fire is cooled down, the 

 engineer should blow off the water freely. This will discharge all the sedi- 

 ment which has been precipitated, and prevent it hardening and adhering to 

 the metal. 



Although this method of working boilers will prevent scale, if the water 

 only contains carbonate of lime, it Avill not entirely suffice to prevent incrus- 

 tations when the sulphate of lime is the principal ingredient in the water, 

 because it does not precipitate like the carbonate. Having by analysis dis- 

 covered the quantity of the sulphate of lime in each gallon of the water to 

 be used as feed, a sufficient quantity of the carbonate of soda is to be em- 

 ployed to neutralize the sulphate and convert it into the carbonate. The car- 

 bonate of soda dissolved is to be fed regularly into the boiler by a pipe con- 

 nected with the water feed-pipe. On land boilers, the carbonate of lime thus 



o* 



