Chemistry and Physics. 445 



filtered off, the alcohol is boiled off, ammonium nitrate, best in 

 the solid state, is added. This precipitates stannic hydroxide, 

 and after the ammonia has been boiled off the precipitation is 

 complete. From the filtered liquid NH 4 MgAsO„ may now be 

 precipitated as usual. It is stated that in this manner 1 mg. of 

 arsenic, 1 mg. of antimony, and 2 mg. of tin can be detected with 

 certainty in 1 g. of copper or lead. — Zeitschr. anorgan. u. allgem. 

 Chem., xcii, 168. h. l. w. 



4. An Alleged Allotropic Form of Lead. — Hans Heller has 

 found that when thin strips of lead are placed for 3 days or more 

 in a solution containing 1000 cc. of water, 400 g. of lead acetate, 

 and 100 cc. of nitric acid of 1*16 s. g., the metal loses completely 

 its original solidity and ductility and falls into small gray to gray- 

 ish-black particles which can be rubbed to a powder between the 

 fingers. It was found that the presence of a lead salt in solution 

 was necessary to produce this change, and the conclusion was 

 reached that the transformation is analogous to the well-known 

 change of white tin into gray tin. ,It was found impossible, how- 

 ever, to change ordinary lead into the gray product by inocula- 

 tion with the latter, so that it is evident that the analogy to tin is 

 not complete. Although it is stated that the change is not a 

 chemical one but an actual transformation of lead into a new 

 modification, this does not seem entirely- plausible, and it appears 

 that a further study of the matter is desirable. — Zeitschr. physi- 

 Jcal. Chem., lxxxix, 761. h. l. w. 



5. Reflection of Gas Molecules. — Certain phenomena asso- 

 ciated with the flow of gases have led to the assumptions that, on 

 collision with a solid wall, the gas molecules are reflected at 

 angles which are independent of the angle of incidence, and that 

 the number of molecules reflected in any given direction is pro- 

 portional to the cosine of the angle which this direction makes 

 with the normal to the reflecting surface. An attempt to verify 

 these hypotheses by direct experiment has been recently made by 

 R. W. Wood. 



The first experiments related primarily to the production of a 

 one-dimensional flow of gas. The apparatus consisted essentially 

 of a vertical glass tube with a small bulb at the lower end and a 

 constriction just above the bulb. A short segment of the upper 

 portion of the tube was bent through an angle of about 130° so 

 as to form a convenient reservoir. In short, the apparatus had 

 the genei'al appearance of the ordinary form of cryophorus. 

 A globule of mercury was first introduced into the bulb and then 

 the apparatus was exhausted to as high a degree as possible. 

 During the process of evacuation the tube was heated with a 

 Bunsen flame to drive out the occluded gases from the glass walls. 

 After the apparatus had cooled sufficiently the bulb alone was 

 heated, thus causing the mercury to condense in the upper por- 

 tion of the tube. The pumping was continued until all of the 

 mercury had been driven out of the bulb and the lateral reservoir 

 had been sealed by fusion. By simple manipulation the mercury 



