254 THE BOOK OF THE LANDED ESTATE. 



and of a whiter colour than any other. Sand which contains earthy 

 matter should not be used ; this may easily be known by rubbing a 

 small portion between the finger and thumb, and if it leaves a dark stain 

 it is not fit to use without being washed. 



8. Cements. These are usually made from argillaceous stones, but 

 sometimes are found in a natural state. They are very valuable in many 

 cases, and especially in forming hydraulic lime. The principal cements 

 in use with builders are Portland cement, and gypsum, or plaster-of-Paris. 

 The latter is generally used for stucco-work. It hardens very rapidly ; 

 but it must only be used in a dry situation, as it absorbs water and then 

 cracks. An artificial hydraulic cement is made by mixing equal por- 

 tions of lime, cement, and sand. In doing this, care must be taken to 

 have the lime made very fine before adding the cement and sand. In 

 fact, to get it made fine enough it is usual to grind it in a mill made for 

 the purpose. 



A good cement for repairing tanks is made by mixing one part of 

 lime to two parts of chalk ; and another, for filling up cracks in iron 

 boilers, is made by mixing one part of iron-filings and five parts of clay, 

 and mixed to a paste with oil. 



The following are the prices of some cements : 



Portland, 2s. 3d. per bushel. 



Roman, ..... Is. 4d. 

 Blue lias, Is. 6d. 



9. Concretes. Concretes are now much used for building purposes, 

 especially in forming the foundations. 



I have found the following composition form a good concrete : 



Cement, 0.30 



Sand, middling 0.25 



Coarse gravel, 0.25 



Broken brick or stone-chips, .... 0.40 



Hydraulic lime, unslaked, . . ... . 0.35 



The sand, lime, and cement are first mixed up together as minutely as 

 possible, so as to make apparently one mass. This is then allowed to 

 remain -for a day, after which it is spread out thinly, and the stones, 

 broken brick, and gravel are mixed with it thoroughly, when it is used. 

 It must be used quickly, as it very soon sets hard. Concrete is exceed- 

 ingly useful in filling in about the foundations of buildings ; it prevents 

 damp from rising, and assists in securing a solid foundation. I have 

 also used common gas-tar with advantage in the foundations of buildings 

 in damp situations, by applying it thinly over the concrete when it was 

 set hard. 



