458 



Transactions. — Chemistry. 



As may be readily imagined it was a great source of trouble and expense 

 to keep the pumps at work while the incrustation was forming inside them, 

 and now that it has ceased forming there, I hope we shall never have a 

 repetition of it. I have sent this slab thinking it may find a place in the 



Museum. 



Note. — Specimens of this incrustation were sent to the Colonial Laboratory 

 in 1875, and reported on as follows : — 



"No. 1753 is an incrustation taken from the cylinder of that large pump 

 used at the Thames diggings for draining the lower levels of certain claims 

 there adjoining the beach. It appeared to have incrusted a portion of this 

 cylinder evenly over to a thickness of about one-quarter of an inch, and is 

 very hard, also impervious to water, and presents to the naked eye an amor- 

 phous appearance, except on its inner side, this being, on the other hand, 

 manifestly semi-crystalUne ; and besides is interspersed, though somewhat 

 rarely, with crystals of iron pyrites. The pyrites is in all probability how- 

 ever, merely a mechanical deposit. 



" The annexed results of an analysis made of a portion of this in- 

 crustation show it to be essentially carbonate of lime. As it has certainly 

 been deposited in this form from some solution of it, and as the solvent for 

 it in this case has most probably been water charged under a considerable 

 pressure with carbonic acid it would appear that this incrustation has been 

 induced by the escape of carbonic acid, caused by reducing the pressure and 

 therefore the capacity of the water for carbonic acid by the action of the 

 pump, resulting of course in a lowering of its solvent power for carbonate of 



lime : — 



Analysis. 



uaroonaxe oi iiiue . . 

 Carbonate of magnesia 



•84 



Iron oxides, witli alumina 



.. 6-69 



Siliceous matters, insoluble in weak acid 



.. 2-18 



Soluble silica 



•44 



Water .. .. 



.. 2-17 



Alkalies, sulphur, etc. 



1-14: 



100^00 " Ed. 



