September 11, 1908] 



SCIENCE 



325 



which is probably manufactured in larger 

 quantities and goes into more industries 

 than any other chemical. As we noted 

 above, it is sold in about five strengths of 

 65, 78, 93, 98 per cent, strength, respective- 

 ly, and fuming acid in addition. All these 

 grades of acid must be made, concentrated 

 and shipped, and the first and most im- 

 portant question to be decided is the kind 

 of material to use at each separate stage 

 of the process. 



You are all no doubt perfectly familiar 

 with the general features of the process 

 of manufacturing sulphuric acid by the 

 chamber and the contact methods, using 

 either brimstone or pyrites. At the risk of 

 wearying you, I am going to take 'some 

 time to sketch through these processes, 

 keeping in mind continuously the extreme 

 importance of a knowledge of the action 

 of chemicals on materials used in the con- 

 struction of apparatus. When sulphur or 

 pyrites is used as the raw material a large 

 amount of cast iron enters into the con- 

 struction of the burners and very often 

 they are made entirely of cast iron, this in 

 spite of the fact that one of the first fresh- 

 man experiments proves that when sulphur 

 is burned in the presence of iron, the 

 products unite to form a sulphide of iron. 

 When brimstone is burned in cast-iron 

 burners this same action takes place to a 

 certain extent, but the coating of sulphide 

 of iron formed protects the rest of the iron 

 from injury. 



When sulphur is burned in the form 

 either of brimstone or of pyrites the greater 

 part, of course, forms SO,, but about 6 per 

 cent, burns to SO3. Either of these two 

 gases may be moved continuously and with- 

 out danger in cast-iron pipes regardless, 

 within reasonable limits, of how hot they 

 become. On the other hand, a certain 

 amount of moisture, depending on the day 

 and the location of the plant, passes in 

 with the air used for combustion (unless 



especial precautions have been taken to dry 

 it all) and appears in the S0„ gases. If 

 these gases are allowed to cool the moisture 

 combines with SO3 in the gas to form di- 

 lute H2SO4, which destroys cast iron very 

 rapidly. At the point where the gases are 

 cooled in the contact process, or enter the 

 glover tower in the chamber process, the 

 iron must be abandoned, and all the cast- 

 iron pipes must be arranged to drain for- 

 ward and not backward. This same 

 feature comes up in making nitric, muri- 

 atic acetic and hydrofluoric acids, all of 

 which attack iron rapidly, and still all are 

 made in iron retorts or furnaces. The 

 necessary precaution, of course, is that no 

 part of the retort or furnace should be 

 allowed to cool enough to permit any con- 

 densation to form liquid products. If the 

 retort is not properly insulated at any 

 place a hole eats through with amazing 

 rapidity. 



Returning to the sulphur dioxide gases, 

 then, we must change from cast iron to lead 

 as soon as the temperature goes low enough 

 to permit condensation. This material is 

 not attacked by weak sulphuric acid. We, 

 of course, could not use lead all the way 

 back to the furnaces, as the excessive heat 

 would melt it. In the chamber system the 

 gases must now continue in lead through- 

 out. In the contact system the sulphuric 

 acid is filtered out after cooling and the 

 gases continue their course in cast iron 

 and steel to the very end without danger. 



In concentrating H^SO^ still more in- 

 teresting details arise regarding the ma- 

 terials that may be used. Many have 

 been suggested, and among others glass and 

 porcelain have been used with more or less 

 success. But a factory is no place for 

 glass and porcelain apparatus if any other 

 materials can be found. The four ma^ 

 terials now in fairly common use are lead, 

 cast iron, volvic lava and platinum. Lead 

 may be used so long as the acid does not 



