MECHANICS AND USEFUL ARTS. 51 



upon dolomite and the usual limestones employed in public 

 buildings. This chemical action, aided by the simultaneous at- 

 tack of carbonic acid and moisture, and by the disintegrating 

 effects of frost, will explain the fact of decay in stone. The best 

 classes of coal contain more or less sulphur; and it has been es- 

 timated that a ton of coal of ordinary quality evolves during its 

 combustion nearly 70 pounds of sulphuric acid. This is the origin 

 of the sulphates always found in the loosened crust of calcareous 

 and magnesian stones when in a state of decay. In dolomites the 

 disintegrated stone often exhibits white ciystals of sulphate of 

 magnesia, which, alternatejy dissolving and recrystallizing in the 

 pores, exerts a disruptive action sufficient to fracture and scale 

 off" the stone. 



Mr. Spiller has succeeded in remedying the difficulty to a con- 

 siderable extent by applying to the cleaned surfaces an aqueous 

 solution of superphosphate of lime, a salt having a remarkable 

 effect in hardening calcareous stones, acting upon the carbonate 

 of lime and forming crystallized diphosphate of lime. Another 

 method, especially applicable to dolomites, is to employ baryta 

 with the hardening salt, so that a base may be presented capable 

 of destroying the soluble sulphate of magnesia in the pores of the 

 stone, forming with it the remarkably insoluble sulphate of baryta, 

 and at the same time engaging the magnesia in one of its most 

 difficultly soluble combinations. Stone thus treated acquires an 

 increased strength to resist a crushing weight of nearly 50 per 

 cent. ; it is also very much harder and much less porous. The 

 cost is trifling, 1 gallon of the solution being sufficient to cover 

 300 superficial feet, when two coatings are applied ; the super- 

 phosphate must not contain any appreciable amount of sulphuric 

 acid, and the specific gravity of the solution, when diluted for 

 use, should be about 1,100. 



PRESERVATION OF STONEf. 



This subject, which has attracted the attention of many chem- 

 ists, seems now to have been brought to a very successful point 

 by Messrs. Dent and Brown, of Woolwich, England. Their pro- 

 cess consists in the application of a solution of oxalate of alumina 

 to the stone. The experiments date from December, 1865, and 

 the results they have now obtained are most encouraging. The 

 process is applicable to limestone, dolomite, and chalk, and may, 

 perhaps, be made subservient to the preparation of lithographic 

 stones. Oxalate of alumina is readily soluble in water, and the 

 solution, which is simply applied with a brush, is made of a 

 strength varying with the porosity of the material to which it is 

 to be applied. The physical characteristics of chalk so treated 

 are lightness, the possession of a glazed surface approaching 

 somewhat, in appearance, marble, and greatly increased hardness ; 

 in this respect, the stone is about equal to fluor-spar. Further- 

 more, the lime being transformed into one of the most insoluble 

 and unalterable of its compounds, and the alumina being precipi- 

 tated, the pores are filled with a substance almost unacted upon 



