326 



SCIENCE 



[N. S. VoL.XXVIII. No. 715 



exceed SO per cent, in strength and so long 

 as direct heat is not applied. Platinum is 

 satisfactory while the acid is between 80 

 per cent, and 94 per cent, strength, but the 

 installation is very expensive and the loss 

 of platinum is very large if an attempt is 

 made to concentrate above 94 per cent. 

 Cast iron is very satisfactory between 88 

 per cent., and 98 per cent. Below 80 per- 

 cent, its life is too short to be considered. 

 Cast iron is always attacked in any case, 

 but it is usually cheaper to use cast-iron 

 pans for six months or so and then replace 

 them than it is to make more expensive in- 

 stallations. Fortunately only a small pro- 

 portion of the iron dissolved by the acid 

 from the pans remains in solution in the 

 acid; nearly all of it is in the form of a 

 sulphate of iron almost insoluble in either 

 water or sulphuric acid, so it readily 

 settles out, leaving a clear acid. 



Where it is desired to produce 98 per 

 cent, acid by concentration, a common cus- 

 tom is to evaporate in lead pans by waste 

 heat to 78 per cent., then in platinum stills 

 by direct heat to 93 per cent., then in cast- 

 iron stills to 98 per cent, Wliat evapo- 

 rates from the lead pans is mainly water 

 and is disregarded. The evaporation from 

 the platinum stills forms a 35 percent, acid, 

 which can, of course, be condensed in lead. 

 That from the cast-iron stills is over 90 

 per cent, and must be condensed in plati- 

 num. If the acid to be evaporated eon- 

 tains much sediment, as is liable to be the 

 case when it is made from pyrites, or if 

 the platinum stills are allowed to become 

 dry and burn, they may be nearly ruined 

 in a single night by a careless workman. 

 The only material to use in mending the 

 leaks which frequently occur in the plati- 

 num stills is gold. This seems rather 

 startling when we consider that the acid 

 itself is sold in quantity for less than a 

 cent a pound, while the brimstone from 



which it is made costs more than a cent 

 a pound. 



Another common and satisfactory ar- 

 rangement where it is only desired to con- 

 centrate up to 93 per cent, strength, is to 

 evaporate in lead pans by waste heat up 

 to 80 per cent, and feed this 80 per cent, 

 acid directly into a large cast-iron still of 

 such capacity that the whole body of acid 

 in it always averages over 90 per cent. 

 This prolongs the life of the iron often to 

 two or three years, instead of its usual life 

 of three to six months. 



The distillate from such an iron still can, 

 of course, be condensed in lead, but the 

 upper part of the still itself would soon 

 be eaten out by the weak distillate unless 

 it were lined with acid brick or repressed 

 red brick set in Portland cement neat, or 

 mixed with silica or fine asbestos. Port- 

 land cement will stand the action of hot 

 weak sulphuric acid for a very considerable 

 time. A still of this type, if properly cared 

 for, usually meets its end from cracking 

 rather than from eating through. This is 

 due to the caking of the peculiar sulphate 

 of iron, which I mentioned above, on the 

 bottom of the still to such an extent that 

 the heat cracks the iron. When such an 

 accident occurs there is, of course, a delay 

 in operation, which is always expensive, 

 aside from the cost of the new still and 

 possibly a loss of several tons of acid. 



Concentration in apparatus made of 

 volvic or volcanic lava rock by driving hot 

 furnace gases directly over and through 

 the sulphuric acid is very satisfactory. 

 The cost of installation is large, but not so 

 large as that of platinum. The lava rock 

 tised comes from a mountain near Cler- 

 mont-Ferrand in central France and so far 

 no satisfactory substitute for it has been 

 found in this country, although a number 

 have been tried, among them some of the 

 hard red sandstones of New York. In the 

 volvic lava concentrating apparatus the 



