332 LECTURES TO SCIENCE TEACHERS. 



reaction can be most completely kept under control, and all 

 irregularities and, therefore, chances of escape of acid vapours 

 rigidly prevented. How far this proposal can be practically 

 carried out remains I believe yet to be ascertained, but I am 

 sure we must all wish success to the ingenious inventors of 

 what I may truly say is the most scientifically correct proposal 

 yet made for the manufacture of salt-cake from sulphuric 

 acid and common salt, 



I daresay you may be interested to learn the principles 

 upon which the Government Inspectors of alkali-works 

 determine the question as to whether a given manufacturer 

 of salt-cake is complying with the Acts passed for the pre- 

 vention of the pollution of the air, and the destruction to 

 property which takes place when hydrochloric acid gas is not 

 properly condensed. I can, however, only indicate the 

 methods adopted, and for particulars on this, and many 

 kindred subjects of great interest, I must refer you to the 

 valuable series of annual reports issued by Dr. E,. Angus 

 Smith. 



The inspector has to ascertain whether the manufacturer 

 is working in compliance with the Acts that is (1) whether 

 he is sending out less than 5 per cent, of his total make of 

 hydrochloric acid ; and (2) whether the gaseous products 

 passing out into the air contain more than one-fifth of a grain 

 of hydrochloric acid per cubic foot. For the purpose of 

 settling the first of these points, he may determine the 

 proportion between the amount of acid gas contained in 

 one cubic foot of the gases passing into the condenser, and 

 that contained in the same bulk of gases leaving the condenser, 

 and thus get to know how many parts of acid escape for 

 every 1 00 which are made. The inspector may also ascer- 

 tain the absolute amount of hydrochloric acid gas which 

 passes up the chimney, and then compare this with the 

 total quantity which the manufacturer evolves, calculated 

 from the total weight of salt which he decomposes. 



The method by which the quantity of hydrochloric acid 

 contained in a cubic foot of chimney gases can be ascer- 

 tained is very simple. We only need to draw a slow 

 current of the gases by means of an aspirator through 

 three bottles containing water, placed one after the other. 

 As soon as one cubic foot of water has flowed out of the 

 aspirator, we know that one cubic foot of chimney gases 

 has passed through our bottles. The whole of the hydro- 



