Intelligence and Miscellaneous Articles. 233 



for about a minute. The iodide of silver first becomes darker, and 

 then milk-white. This white substance is very sensitive to light, 

 and is in this respect little inferior to any known. By exposure to 

 light, and indeed by all of its colours, it is converted into a steel- 

 gray. The plate must therefore be protected from the direct light 

 of the sky, and the experiment carried on in the back part of the 

 room. When cold it is placed behind a cut-out screen, which may 

 be at the distance of a line from the plate over mercury which is 

 heated to 60° R., and the temperature allowed to fall to 30°. When 

 the plate is now removed, it has become steel-gray wherever the 

 vapour of mercury had access, and in this manner the image of the 

 aperture of the screen is obtained precisely as if ordinary light had 

 fallen on to the plate. Although the condensed vapour of mercury 

 is white, yet the action of its latent light preponderates in this case 

 and determines the colouring. 



Heat acts no part here, for it has not the power of rendering the 

 white substance steel-gray ; nor can there be any question of chemi- 

 cal rays with this white substance, for all the rays of the spectrum 

 convert it into steel-gray. — Konigsberg, July 1843. 







ON THE EQUIVALENT OF ZINC. BY MONS. P. A. FAVRE. 



The author remarks that the hypothesis of Dr. Prout, submitted 

 to experiment by M. Dumas, has become in his hands a subject of 

 the highest importance. The experiments published by M. Jacque- 

 lain to determine the equivalent of zinc induced M. Favre to under- 

 take the subject ; the conviction expressed by M. Jacquelain, that 

 the number 414, stated by him, is a minimum, would inevitably re- 

 move zinc from the series of the multiples of hydrogen. 



To clear up this subject M. Favre analysed several specimens of 

 oxalate of zinc prepared with the greatest care, and he also deter- 

 mined the quantity of water decomposed in oxidizing a given weight 

 of zinc. The gaseous products of the decomposition of oxalate of 

 zinc were passed over oxide of copper heated to redness, and the 

 carbonic acid formed was condensed ; knowing the weight of this, 

 and the corresponding weight of the residual oxide of zinc, all the 

 requisites for determining the equivalent of zinc are obtained, that 

 of carbon being already known. This mode of experimenting has 

 the advantage of supplying all the elements for calculation by one 

 operation only ; besides which it allows of deducting the accidental 

 water which the salt may contain ; the experiments executed on this 

 plan yielded the following numbers as the equivalent of zinc ; the 

 quantity of carbonic acid obtained was, in some cases, from about 

 123 grains, of oxalate, and never from less than 77 grains. 



I. II. III. IV. Mean. 



412-58 412-25 413-36 412'45 412-66 



These numbers lead to the number 33'01 as the equivalent of zinc, 

 that of hydrogen being reckoned unity. 



The second series of experiments was performed by burning, by 

 means of oxide of copper, the whole of the hydrogen obtained by de- 

 composing water with sulphuric acid and zinc, the metal being pure 



