30 PROCEEDINGS OF THE AMERICAN ACADEMY 



possible, about 7 mm. inside and 8 mm. outside diameter. It should 

 be drawn out before a small blast-lamp flame to as nearly as possible 

 definite bore, which ought to be from 1.5 mm. to 2 mm. at the place of 

 deposition of the arsenic mirror. The finely drawn tube is bent 

 slightly upward, and sealed at the end. 



The Reagents (App., 4). — The zinc and sulphuric acid used in the 

 apparatus must be strictly free from arsenic, and one should not feel 

 satisfied of their purity unless a stream of hydrogen from the genera- 

 tor, led through the heated tube for several hours, fails to give the 

 slightest deposit. The granulated zinc used in the generator is best 

 of comparatively large size, while that used in the reduction flasks 

 should be quite fine-grained. The acid should have a concentration 

 of one part strong acid (1.82 sp. gr.) to eight parts of water, though 

 a more dilute acid can often be used. 



The Course of Analysis. — In the reduction flask is placed a small 

 quantity, not over three grams, of zinc, and the apparatus is con- 

 nected together. Tightness must be assured, and can be tested for, if 

 the tip of the reduction tube is sealed, by adding through the small 

 funnel a few drops of acid. If these do not fall, the tip of the tube is 

 broken off, leaving an opening of not more than 1 mm. diameter, and 

 then about 20 c. c. of acid are added. The hydroiiea is now turned 

 on from the generator, aud, after expulsion of the air, lighted, and the 

 flame turned down to a height of 1 to 2 mm. The evolution of 

 hydrogen should be kept at this rate during the reduction of the 

 arsenical solution. Often it is not necessary to use the generator 

 during the reduction, as the evolution from the reduction flask is 

 sufficiently rapid. If it slackens, the generator may be used again, 

 and indeed it is generally necessary to use it toward the close of 

 the reduction. It may happen, on account of too strong acid or 

 increased action after introduction of the extract, that the evolution 

 is too rapid, and the flask becomes heated (App., 5. c). To obviate 

 this, the flask may be set in a vessel fitted with an exit tubulus, 

 and filled with cold water, which can be drawn off" and replaced 

 when necessary without disturbing the apparatus. An ordinary 

 crystallizing dish with a siphon would answer the purpose. 



Shortly after (App., 5. e) lighting the hydrogen, the lamp is 

 placed under the heating place and the apparatus tested for absence 

 of arsenic for such length of time as the circumstances direct. In or- 

 dinary analyses of wall paper, I allow 15 to 20 minutes' free run 

 before adding the extract. 



The lamp should give a large clearly defined flame (App., 5./), 



