Polytechnic Association. 759 



Prof. John Phin — That the hygrometric condition of the atmos- 

 phere may produce spontaneous combustion is well known to chemists. 



VI. A Spiral Leyden Jak. 

 Mr Frederick Guthrie, in a note to the Philosophical Magazine 

 (London), thus describes its construction : A strip of tin-foil four feet 

 long and eight inches wide is placed upon a strip of vulcanized 

 caoutchouc four feet long and one foot wide, in such a way that along 

 both sides there is a margin of four inches of tin-foil. A second piece 

 of caoutchouc, exactly similar to the first, is placed exactly over the 

 first upon the foil. A second piece of tin-foil, of the same width as 

 the first, but four inches shorter, is placed on the second caoutchouc 

 above the first foil, with its right-hand end above the right end of the 

 first foil ; its left end, of course, falls four inches short. A brass wire, 

 carrying a knob, is laid across the end of the upper foil. The whole 

 is rolled up from the right end and bound. What was the lower of 

 the two foils projects between the two layers of caoutchouc, and may 

 be prolonged around the circumference of the roll ; it forms the outer 

 coating or earth-surface. What was the upper coating of foil now 

 forms what corresponds to the inner coating of the ordinary jar, and 

 is entirely covered, excepting where it is prolonged as the wire and 

 knob at the center of the roll. If the sheet of caoutchouc be an 

 inch in thickness, a jar of very great electrical capacity is obtained, in 

 a very compact form, and one which is free from the risk of fracture, 

 and is less impaired than the ordinary jar by atmospheric moisture. 

 A very serviceable modification of this form has been constructed, by 

 using for the insulating material sheet ebonite. The ends of the 

 spiral roll are capped with dry mahogany disks. The earth-foil is 

 connected with a brass girdle around the center of the cylinder, and 

 is not visible. The electric capacity is between four and five times as 

 great as that of a glass jar of the same volume. It has been in use 

 for several months, and appears almost incapable of injury. 



VII. Permanence of Silver Prints. 

 Mr. M. Carey Lea, in a communication to the Philadelphia Photo- 

 grapher, says : At brief intervals the question of the permanence of 

 ordinary silver prints comes up again and again for discussion. The 

 loss, when the fading takes place, is almost always serious, often irre- 

 parable ; and those on whom it falls are apt, and not unnaturally, 

 to express themselves with indignation. He repeats the conviction, 

 often before expressed, that a well-made silver print, on albumenized 



