Chemistry and Physics. 391 



sideration provided that it is certain that the pointer will always 

 return to the same place. It is not intended to criticize adversely 

 this method of weighing, for with the proper precautions it will 

 evidently give good results, but it is believed that observations 

 of the return swing would obviate uncertainties in the action of 

 the pan-arresters, would make the manipulation of the latter 

 less tedious, and would usually increase the ease, rapidity and 

 certainty of weighing. — Jour. Amer. Chem. Soc. 41, 1151. 



h. l. w. 



2. A Method of Growing Large Perfect Crystals from Solu- 

 tion. — E. W. Moore, of the Research Laboratory of the General 

 Chemical Co., has described a very satisfactory method for this 

 purpose. He refers to other methods that have been devised 

 in recent years and mentions the work of J. M. Blake described 

 in an article in this Journal in 1915. Moore has worked chiefly 

 with Rochelle salt, but has applied the process successfully also 

 to potassium alum. A saturated solution of Eochelle salt is 

 made up at a convenient, accurately determined temperature, 

 usually between 35 and 40°. The solution is removed from the 

 excess of salt, heated to about 7 or 8° above the saturation tem- 

 perature, filtered and kept not less than 4 or 5° above that 

 temperature. Small seed crystals are hung in a jar by means 

 of threads or wires fitting into grooves cut around these crystals. 

 Then the salt solution is poured in, the jar is covered with a 

 glass plate and placed in a water bath the temperature of which 

 is about 0-5° above the saturation point of the solution. The 

 temperature is allowed to fall to practically the temperature of 

 saturation as fast as the bath cools off, then by means of a deli- 

 cate thermometer and a thermostat the temperature is allowed 

 to drop at the rate of about 0-1° per day until the crystals have 

 increased noticeably in size and are perfect in form. This 

 usually takes about one day. Then the temperature is lowered 

 at the rate of 0-2° a day until the crystals are about an inch 

 long, and the rate of cooling is gradually increased to 0-3 to 04° 

 and 05 to 0-6° as the crystals become larger. The thermostat 

 setting is changed twice a day. When the solution has cooled to 

 about room temperature the crystals are removed and dried by 

 wiping with a soft, dry cloth. The method requires care for the 

 production of perfect crystals, and about a month was required 

 to grow a clear, perfectly developed crystal about 3 inches long. 

 For details reference must be made to the original article. — 

 Jour. Amer. Chem. Soc, 41, 1060. h. l. w. 



3. Modifications of Pearce's Method for Arsenic. — The origi- 

 nal method of Pearce consists in fusing the substance with 

 sodium carbonate and potassium nitrate, extracting the result- 

 ing mass with water, boiling the filtrate -with excess of nitric 

 acid, neutralizing exactly with ammonia, precipitating silver 

 arsenate, filtering off the latter, dissolving it in nitric acid, and 

 titrating for silver with a thiocyanate solution. Since the 



