Chemistry and Physics. 221 



introduced into a hollow brass sphere polished without and black- 

 ened within, and having an aperture 30 mm. in diameter through 

 which the sun's rays penetrate. The observation of the heating 

 after numerous corrections gives with great delicacy the relative 

 heat of the sun at different times. — Gomptes Rendus^ Ixxxi, 1205. 



U. Thermal Eqnwaleyit of Magnetism.— M. A. Cazin has pub- 

 lished in full a series of experiments on the relations of heat and 

 magnetism. In the first portion of the memoir three methods are 

 described of measuring the relative values of the heat created by 

 the disappearance of magnetism, in the core of an electro-magnet. 

 The second section demonstrates several laws of the magnetic 

 heat developed, and shows that this heat is really due to the dis- 

 appearance oi' the magnetism. But in the induction of the core 

 on the coil, and of the coil on itself, causes of heat are found 

 which should be allowed for. The fundamental law deduced 

 from these experiments is, that the disappearance of magnetism 

 in the core of a bar electro-magnet having two poles is accom- 

 panied by the creation of a quantity of heat Q proportional to the 

 polar interval /, and to the square of the quantity of temporary 

 magnetism m which the core acquires when the circuit is closed. 



^- will be the mechanical equivalent of heat. In the third s 



~Q~ 



tion the value of Q is determined in units of heat while the effects 

 of induction are inconsiderable. The first series gave as a mean 

 of five experiments while the spark was broken in all 110600000, 

 as the magnetic equivalent. A second more reliable series with 

 the spark broken in ether gave 106000000. Both are a little too 

 great because the induced current on breaking the circuit is not 

 zero. Hence, probably the true value does not differ materially 

 from 100000000.— .4w?i. Chim. et Phys., vi, 493-554. e. c. p. 

 12 ~ 



ing c __^ ^ ^ . 



that the charge given to a Leyden jar shall not exceed any fix 

 limit. Through a cork in the upper end of a bell-glass passes a 

 brass rod, insulated through its entire length by a glass tube, 

 through which it passes freely. To the upper end is attached a 

 brass knob, and the lower end is pointed and provided with a 

 screw thread, so that it can be set at any distance within, or 

 through, a hollow brass ball, perforated below and rigidly fixed 

 to the glass tube. Within the bell-glass is a loose cage of perfor- 

 "*"""' • - - ntaining strong sulphu '- "" '*^" 



.„ „„ ,„„ „ .„j rod be screwed down 



that the point projects through the hollow ball, the upper knob 

 and the lower metallic plate being connected with the two poles 

 of a Holtz machine, only short sparks can be obtained, because a 

 large amount of electricity escapes at the point ; but if the rod 



