318 Mr Stevenson Macadam on the 



added to it, and subsequently hydrochloric acid and nitrite of po- 

 tassa, which together form a most delicate means of detecting iodine* — 

 the result was negative. The nitrate of silver solution was cau- 

 tiously evaporated to nearly a quarter of an ounce, a stream of sul- 

 phuretted hydrogen passed through to precipitate the silver, and 

 liberate as hydriodic acid any iodine which might be present — the 

 liquid raised in temperature, carefully avoiding ebullition, and fil- 

 tered. The filtrate, on the addition of starch, hydrochloric acid, 

 and nitrite of potassa, did not exhibit the slightest trace of iodine. 

 I therefore concluded, that in the large volume of air — upwards of 

 300 cubic feet — which had been drawn through the arrangement, 

 there had not been an appreciable amount of iodine, either in a free 

 state (in which case the starched paper would have been acted upon), 

 or combined with a metal or base (in which condition it would have 

 been detained by the nitrate of silver, forming iodide of silver). 



The experiments referred to, were made at different heights 

 on Arthur Seat, and their negative results led to arrangements 

 being made for a trial on a larger scale. Through the kind permis- 

 sion of the proprietor of Kinneil Iron- Works, I was enabled to pro- 

 ceed to Borrowstonness and attach my apparatus to the receiver 

 from which the air under great pressure is forced into the blast-fur- 

 naces. By means of a stopcock fixed in the receiver and a long 

 flexible tube, the air was conducted to the following arrangement : — 



1. A wide tube, containing slips of paper dipped in starch. 



2. A condensing worm, nine feet in length, surrounded by a 

 freezing mixture, and attached to a receiver. 



3. A tall jar, containing chips of pumice-stone, and a few iron 

 filings, with sufficient water to cover them. 



4. A similar jar, with pumice-stone, scrapings of clean lead, and 

 a solution of acetate of lead. 



5. A condensing worm, nine feet in length, immersed in a freez- 

 ing mixture, and attached to a receiver. 



By this arrangement it was expected that the first condenser (2) 

 would retain the water, vapour, and salts, which the air experi- 

 mented upon held in suspension, and should the accumulated liquid 

 be sufficient to fill the tube, the excess would be projected into the 

 receiver, and thus be kept from passing into other parts of the appa- 

 ratus. The jar (3) was capable of retaining any free iodine, and 

 was intended as an auxiliary to the papers (1). The chips of pu- 

 mice-stone enabled the air, as it gurgled through the several layers, 

 to come in contact with the reagents contained in the jar. The 

 office to be fulfilled by the solution of lead in jar (4) in retaining 



* Quarterly Journal of the Chemical Society, vol. iv., p. 155. Dr Price 

 says, " In this manner I have detected the frjfjwggtb part of iodine dissolved in 

 water, as iodide of potassium. ' ; 



