THE WOMBAT. 87 



and swells, gives off a quantity of hot vapour, and quickly falls 

 to pieces. It increases two or three times in bulk, and if 

 placed in a vessel of water, it falls [to the bottom and always 

 remains soft. 



2. Poor lime does not increase in bulk when water is 

 poured over it, in fact water scarcely affects it at all ; neither 

 does it harden in water. 



3. Intermediate or hydraulic lime develops no heat 

 when water is poured over it, nor does it increase in bulk ; 

 when, however, it is placed in a vessel of water it becomes a 

 hard solidified mass after a few days, and will continue to 

 harden for months. 



To summarise these classes, we have 



1. The fat or rich, which does not harden in water, and 

 which will contain over 90% of carbonate of lime. 



2. The poor, which is in no way affected by water, and 

 contains less than 50% of carbonate of lime. 



3. The intermediate or hydraulic, which when burnt and 

 ground sets in a hard mass when put in water, and contains 

 from 7o% to 80% of carbonate of lime. 



Smeaton is undoubtedly the engineer to whom we are 

 mostly indebted for our knowledge of hydraulic lime His 

 great work in rebuilding the Eddystone Lighthouse necessitated 

 him using a lime that would set and become hard in water, and 

 up to that period it was a somewhat doubtful point as to which 

 limes did set in water ; one of the commoner ideas was that 

 the harder the limestone, the harder the mortar would ultimately 

 become. Smeaton first tried experiments with a rich lime of 

 undoubted repute for building purposes, but he found that 

 when it was exposed to the action of water it was quite useless. 

 During his searches in the various lime districts he stayed one 

 day with a farmer who told him that the lime in his district 

 was quite unfit for agricultural purposes, and what they used 

 had to be carted a distance of forty miles. On enquiring as 

 to the reason he was told that when the local lime was ground 

 and put on the land it became quite hard after the first shower 

 of rain. This was sufficient to give Smeaton a clue to work 

 on, and he was not long in carrying out a series of experiments, 

 which satisfied him that it was only the intermediate or hydraulic 

 lime which became hard in water. He soon found that lime- 

 stones containing about 70 parts of carbonate of lime were best 

 suited for hydraulic purposes, and that the remaining 30 parts 

 were an argillaceous clay. Further experiments were 

 then made to see if it were possible to make an artificial 

 hydraulic lime, and for this purpose a fat rich lime was taken 

 and clay added until it became an intermediate lime — in other 

 words, clay was added to a limestone of 90% until it only 



