379 



But sulphate of lime, like all difficultly soluble salts, 

 requires an appreciable portion of time for its solution ; and 

 before the fragment left by one bubble is dissolved, myriads 

 of others are formed, which even a weak solution cannot 

 take up before they are aggregated into masses which present 

 less surface, and still further resist the solvent action of the 

 water. Hence they may be, and I doubt not are, deposited 

 from a solution briskly evaporated, but considerably below 

 saturation. 



Carbonate of magnesia accompanies carbonate of lime 

 in many waters, and is subject to nearly the same laws ; it 

 is insoluble in water alone : is soluble by the aid of excess 

 of carbonic acid, and precipitated when this is disengaged 

 by heat. But sulphate of magnesia, being a salt very easily 

 soluble, does not separate in the indirect and curious manner 

 in which sulphate of lime separates from weak solutions. 



In many crusts which I have examined, I have found a 

 mixture of these three substances, carbonate of lime forming 

 always the largest part, sometimes almost the whole ; car- 

 bonate of magnesia is sometimes present, sulphate of lime 

 frequently. 



Carbonate of lime, in native crystals, is harder than sul- 

 phate of lime ; calc spar scratches gypsum, and is not readily 

 scratched by it. In the loosely coherent state in which these 

 salts are at first deposited in a boiler, the true hardness of 

 each minute crystal is no guide to the hardness of the aggre- 

 gate. And I am decidedly of opinion, from my observations, 

 that sulphate of lime is more disposed to attach itself to 

 foreign bodies, as the plates or tubes, than carbonate ; and 

 that a mixture of sulphate gives a closer and firmer texture 

 to the mixed deposits, than is possessed by the carbonate 

 alone. I therefore look with jealousy on all waters con- 

 taining much sulphate of lime, whether it be accompanied 

 by a large or small proportion of carbonate of lime. 



