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LECTURES TO SCIENCE TEACHERS. 



of 1874, further declared that no alkali- works shall emit 

 hydrochloric acid gas of such strength as to contain more 

 than 0'2 grain (one-fifth of a grain) in every cubic foot. So 

 that the alkali-maker is now compelled not only to condense 

 95 per cent, at least of all the hydrochloric acid he makes, 

 but he is not allowed to emit that 5 per cent, unless it 

 shall be so diluted with air or other harmless erases that 



one cubic foot of the emanating gas contains less than one 

 fifth of a grain of hydrochloric acid. 



We will now proceed to consider the various parts of 

 Leblanc's process, and discuss the chemical changes which 

 here occur. 



1. The Salt-cake Process. This process is usually com- 

 menced in large cast-iron pots, and completed in rever- 

 beratory furnaces or roasters. Figs. 8 and 9 show the 0011- 



FlG. $. 



struction of one of the forms of salt-cake furnace in use. It 

 consists of the large covered iron pan (a) Fig. 9 placed in 

 the centre of the furnace and heated by a fire placed under- 

 neath, and two roasters (dd) placed one at each end, on the 

 hearths of which the salt is completely decomposed. 



The charge consists of 1 6 cwt. of common salt, which is 



