OP ARTS AND SCIENCES. 41 



b. Impurities in the Glass of the Deposition Tube. — The errors le- 

 sulting from the presence of lead and arseuic in glass have been fre- 

 quently mentioned. The presence of lead in any hard glass fit for 

 use is scarcely probable. The formation of a mirror from either of 

 these sources would however be detected at the start, and the glass 

 rejected at once. 1 have not found any case of error attributable 

 to impurities in the glass. 



c. Temperature of the Bediiction Flask. — Dragendorflf* quotes 

 Kolbet as having shown that sulphuretted hydrogen is always 

 formed by the action of sulphuric acid on zinc, if the reaction tem- 

 perature exceeds 30°, and recommends on this account that the flask 

 be cooled. The quotation is misleading. What Kolbe showed, and 

 Fordos and Gclis % called attention to the same point some time 

 before Kolbe, was, that if strong acid be added to the flask to in- 

 cf-ease the action, there was reduction of the acid at the temperature 

 mentioned. With acid diluted with two parts of water no reduction 

 took place. As one would hardly add strong acid to the flask, the 

 danger of formation of sulphuretted hydrogen from mere action of the 

 acid on the zinc is not to be feared. Yet it is necessary to keep 

 the flask cool in some such manner as suggested, if only to prevent 

 too violent action of the acid which may occur from accidental pres- 

 ence of organic matter. 



d. Necessity of boiling the Char with Water. — The formation of a 

 " sulphur mirror " is often met with, due to the decomposition of sul- 

 phuretted hydrogen at the heating place. Then, too, Brunn § has 

 lately shown that sulphuretted hydrogen and arseniuretted hydrogen 

 form, when heated, hydrogen and arsenious sulphide. The necessity 

 of boiling- the char with water is therefore evident, in order that no 

 sulphur dioxide be left in the extract to be reduced by the nascent 

 hydrogen. 



e. Beginning of Heating. — If the tube be heated as soon as the 

 hydrogen is lighted, the small amount of oxygen left in the flask 

 causes the formation of water in the deposition tube. This can be 

 avoided, if desired, by waiting a few minutes before setting the lamp 

 under the tube. 



f Rapidity of Gas flow and Amount of Heat. — In the method of 

 Chittenden and Donaldson it is necessary to guard against too rapid 

 evolution of gas, and the heating surface must be very great in order 



* Loc. cit., p. 336. t Ann. d. Chem. u. Pharm., OXIX. 174. 



X Comptes Rendus, XIII. 437. § Ber. d. deutsch. cliera. Gesell., XXII. 3202. 



