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occasions, when my subject required, called attention to its 
presence, and expressed an opinion, which I have found much 
to confirm, that it is even more troublesome and mischievous 
than the carbonate alone. The specimen before you was 
formed, under somewhat peculiar circumstances, in a low- 
pressure boiler. It contains not a trace of carbonate, yields 
not a bubble of effervescence with acids, and its solution in a 
large quantity of water yields, with chloride of barium, a 
quantity of sulphate of barytes, closely equivalent to what it 
would furnish if pure anhydrous sulphate of lime. It contains 
a little iron. It is not the curious salt discovered by Professor 
Johnston, containing half an atom of water to each atom of 
sulphate of lime, (for ten grains, finely powdered, lost by 
exposure to a red heat, only three tenths of a grain, — less 
than a quarter of an atom of water), and therefore, hygro- 
metric, or accidental; and the sulphate is essentially anhy- 
drous. The deposition of sulphate of lime from a solution 
far below saturation takes place in a manner which I described 
some years ago, in the Journal of the Royal Institution. 
As each bubble of steam is disengaged during brisk ebullition, 
the sulphate of lime of course separates ; for its re-solution 
time would be required, but before that can take place many 
other particles are separated, and these rapidly cohere into 
portions large enough to subside, and to resist yet more the 
solvent power of the water. I have elsewhere, and on other 
occasions, stated my belief that though gypsum, in its hydrous 
and ordinary crystals, is a softer mineral than calc spar, yet 
that boiler crusts containing much sulphate of lime are harder 
than those composed wholly or chiefly of carbonate. The 
present specimen curiously confirms this opinion. I am 
assured by the workmen that not only is it with difficulty 
removed by the tools usually employed for such purposes, 
but that even the " Sate," or hard chisel used for cutting 
cold iron, is sometimes broken or turned by this crust. 
