THE MANAGER ON THE WORKS, 85 



the last box and no material ready or in fit condition to go 

 in. Take care to keep a step ahead, by always having one 

 charge in readiness, and as soon as it is used, bring up 

 another. 



The efficient preparation of purifying material, whether 

 lime or oxide, is not a matter that can be learnt from 

 books. Some practical experience is necessary, in order 

 to get the maximum of work from the minimum of material. 

 Cheap and effective purification is not to be done with 

 matchboxes, but with vessels of a sufficient size ; and the 

 areas usually quoted in text-books may advantageously be 

 exceeded in works having a maximum of less than 

 25,000 cubic feet per diem. If lime is the material 

 used, it should be known that a good building 

 lime is not necessarily a good material for the puri- 

 fication shed. The ingredient required is chemically 

 pure lime, Ca 2 O, and carbonated lime (incompletely burned 

 lime) or silicate (over-burned lime) is as valueless as a brick- 

 bat. So, if there is any doubt about the quality of a lime, 

 it is well to get it analyzed for percentage of Ca 2 O. A good 

 lime slakes actively, swells to quite double the original size, 

 and breaks up and crumbles to a coarse powder. If it does 

 not come up to this standard, but is sluggish and cold in 

 slaking, does not swell or crumble, or retains hard fragments 

 that absolutely refuse to break up, it is insufficiently or 

 improperly burned, and inferior in respect to percentage of 

 Ca 2 O. A good lime will not need passing through a screen. 

 It is sufficient to throw it up against a wall, and to break 

 up the lumps that roll to the bottom edge of the heap. It 

 should retain a considerable quantity of water in addition 

 to that which combines chemically with the lime, and the 

 more it will take, without becoming sodden or soppy, the 

 better. It should be moist enough to cohere when squeezed 



