Polytechnic Association Proceedings. ggl 



nide of potassium, and then dries it. For iron filings he substi- 

 tutes pulverized magnetite or loadstone, which is placed upon the 

 paper and submitted to the action of a magnet. After a beautiful 

 combination of curves, called the magnetic phantom, have been 

 made, pure hydrochloric acid gas is brought in contact with the 

 paper and figures, and, after a few seconds, removed. The paper, 

 being then freed from the loadstone powder, is washed very thor- 

 oughly; when drj', a dark blue figure will show, with great deli- 

 cacy of detail, the positions assumed by the pulverized ore under 

 the influence of the magnet. The method is said to be more con- 

 venient and exact than the plan of covering the paper with a 

 mixture of gelatine and starch, or parafiine and wax. 



STABILITY OF GUN-COTTON. 



F. A. Abel, F. R. S., has published the results of his experiments 

 with Von Lenk's gun-cotton, extending over a period of three and 

 a half years, which differ materially from those obtained by 

 Pelouze and Maury, who came to the conclusion that, after expo- 

 sure to light and heat, this kind of gun-cotton was liable to spon- 

 taneous decomposition and explosion. Mr. Abel, however, admits 

 that the ordinary gun-cotton contains small proportions of nitrogen- 

 ized impurities, having instable properties, which are formed by 

 the action of nitric acid upon foreign matters contained in the 

 cotton fiber. These impurities give rise to free acid, which may 

 be efiectually neutralized and rendered nascent by introducing 

 into the gun-cotton, when first made, one per cent of carbonate of 

 soda. To gun-cotton which has not been long exposed to the action 

 of sunlight, water acts as a perfect protector. If it contains suffi- 

 cient to feel damp to the touch, it is perfectly non-explosive, and 

 while in this condition it may be stored in large quantities or trans- 

 ported to very distant places. The best practice would be to dis- 

 solve the required proportion of carbonate of soda in water, and 

 keep the cotton constantly damp with this solution. 



SUN SPOTS AND METEORIC SHOWERS. 



Mr. Hofratn Schwabe has noticed a connection between these 

 two classes of phenomena. He finds that the minimum of spot- 

 frequency coincides remarkably with the recurrence of meteoric 

 showers, the period of rotation of which, viz: 33.2 years, agrees 

 with a larger period of the sun spots. In 1833, there was an 

 extreme scarcity of spots (only thirty-three very small groups being 

 observed), and in 18$6-7, after thirty-three years, the phenomena 



