268 Scientific Intelligence. 



m 



that of bismuth and the reverse of that of iron. When a bar of iron is 

 set erect, its lower end is known to be a north pole, and its upper end a 

 south pole, in virtue of the earth's induction. A marble statue, on the 

 contrary, lias its feet a south pole, and its head a north pole, and there is 

 no doubt that the same remark applies to its living archetype ; each man 

 ■walking over the earth's surface is a true diaraagnet, with its poles the 

 reverse of those of a mass of magnetic matter of the same shape and in 

 a similar position. 



An experiment of practical value, as affording a ready estimate of the 

 different conductive powers of two metals for electricity, was exhibited, 

 for the purpose of proving experimentally some of the assertions made 

 by the speaker in reference to this subject. A cube of bismuth was taken 

 and suspended by a twisted string between the two poles of an electro- 

 magnet. The cube was attached by a short copper wire to a little square 

 pyramid, the base of which w^as horizontal, and its sides formed of four 

 small triangular pieces of looking-glass. A beam of light was suffered 

 to fall upon this reflector, and as the reflector followed the motion of the 

 cube the images cast from its sides followed each other in succession, each 

 describing a circle of about thirty feet in diameter. As the velocity of 

 rotation augmented, these images blended into a continuous ring of light. 

 At a particular instant the electro-magnet was excited, currents were 

 evolved in the rotating cube, and the strenglh of these currents, which 

 increases with the conductivity of the cube for electricity, was practically 

 estimated by the time required to bring the cube and its associated mir- 

 rors to a state of rest. With bismuth this time amounted to a score of 

 seconds or more : a cube of copper, on the contrary, was struck almost^ 

 instantly motionless when the circuit was established- 



5. On M. Petitjeanh process for Silvering Glass; by Professor Fara- 

 day, D.C.L., F.R.a (Proc, Roy. Inst, of Great Britain, vol. ii, p- 308). 

 M. Petitjean^s process consists essentially in the preparation of a solution 

 containing oxyd of silver, ammonia, nitric and tartaric acids, able to de- 

 posit metallic silver either at common or somoAvhat elevated temperatures; 

 and in the right application of this solution to glass, either in the form of 

 plates or vessels. 1540 grains of nitrate of silver being treated with 055 



grains of strong solution of ammonia, and afterwards with 7700 grains 

 of water, yields a solution, to which when clear 170 grains of tartaric acid 

 dissolved in G80 grains of water are to be added, and then 152 cubic inches 

 more of water, with good agitation. When the liquid has settled, the 

 clear part is to be poured off; 152 cubic inches of water to be added to 

 the remaining solid matter, that as much may be dissolved as possible; 

 and the clear fluids to be put together aud increased by the further addi- 

 tion of 61 cubic inches of water. This is the silvering solution No. 1- 

 A second fluid, No. 2, is to be prepared in like manner, with this differ- 

 ence, that the tartaric acid is to be doubled in quantity. The apparatus 

 employed for the silvering of glass plate consists of a cast-iron table box, 

 containing water within, and a set of gas burners beneath to heat it: the 

 upper surface of the table is planed and set truly horizontal by a level, 

 and covered by a varnished cloth : heat is applied until the temperature 

 is 140^ Fah. The glass is well cleaned, first with a cloth ; after which a 

 plug of cotton, dipped in the silvering fluid and a little polishing powder, 



