178 



FIXED STRUCTURES FOR GROWING 



water apparatus ; for hard water deposits a sediment or incrustation, 

 which if not removed, will form a coating of several inches in thick- 

 ness, which coating acting as a powerful non-conductor, will allow the 

 bottom of the boiler to become red-hot without sufficiently heating the 

 water it contains ; and ultimately, from the cracking of the deposit in 

 consequence of the greater expansion of the red-hot iron, the water 

 comes in contact with the red-hot metal, and an explosion takes place. 

 (See ' Gard. Mag.,' vol. ix. p. 206.) Hence the necessity of having all 

 boilers where hard water is to be used constructed so as to admit of 



Fig. 147. 



Weeks s patent duplex 'boiler. 



being readily cleaned out. As the deposit consists of calcareous 

 matter, it may be removed by a weak solution of muriatic acid aided 

 with a slight mechanical agitation : but it is much better to prevent its 

 taking place by using only soft water. 



To prevent the water in the apparatus from freezing, salt may be 

 added to it ; but this may be rendered unnecessary in the case of hori- 

 zontal pipes by drawing off a portion of the water, so that they shall 

 not be quite full, because in that case the water has room for that ex- 

 pansion which takes place when it passes into ice. The quantity of 

 salt put into water to keep it from freezing, Mr. Hood observes, may 



