102 Transactions. 



the ordinary Mahurangi lime, tlie writer v»ras induced to investigate the 

 properties of some limestone from Mahurangi, but obtained on tbe opposite 

 side of tlie river from vv^bence tbat commonly used is procured. 



A fresUy-burnt sample was obtained, as also a portion of the unburnt 

 stone. The latter was analyzed by Mr. Streby, tben in Auckland. Tbe 

 result was, — 



Carbonate of lime 



78-0 



„ iron 



4-2 



„ magnesia ... 



0-4 



Silicate of alumina 



13-5 



"Water 



3-9 



100-0 



The above analysis indicates a moderately hydraulic lime, and it agrees 

 well with the practical tests to which the sample was put. The rock as then 

 worked was, however, by no means homogeneous, so that neither test could 

 be depended on as fairly representing the whole. It will be observed that 

 the silicate of alumina, or clay, is 13'5 per cent, of the whole, and it is now 

 pretty well agreed that it is this which is the active substance in giving 

 hydraulicity to lime. Also, tha,t even in the most highly burnt limes and 

 cements, the lime and silicate of alumina are only mechanically combined 

 until the formation of a hydrate takes place. 



The practical tests to which the lime was subjected were, — 



(1.) The slaking, by immersing a piece in water for about half a minute, 

 and observing the time which elapsed before it became powdery. This was 

 generally about ten minutes. 



(2.) A strong paste was formed by the addition of water, and a saucer 

 was filled and immersed in water. The hardening process was observable in 

 about three Aveeks, and in two months it required considerable pressure to 

 force a penknife into the mass. 



(3.) A portion was made into mortar with common sharp sand, and it 

 exhibited middling setting powers under water. 



(4.) Another portion was mixed with sifted volcanic sand, in various 

 proportions, and made into mortar. This showed a considerable improve- 

 ment in the hardening under water. The best proportions were found to be 

 equal parts of sand and lime. 



The above investigations were undertaken about three years ago, and 

 were mainly instigated by the scarcity of Portland cement in Auckland, 

 £2 to £3 being asked for this per barrel at the time, and some of the 

 samples, to the writer's knowledge, were about as much virorth as so much 

 road dust. The railway was then in progress, and the contractor for the 

 first section had failed to procure a supply of cement in time. The results 



