r 387 ] 

 XXXIX. Intelligence and Miscellaneous Articles. 



k SILVERING GLASS IN THE COLD. 

 BY MM. AUGUSTE AND LOUIS LUMIERE. 



T^OE silvering glass in the cold, a number of methods have been 

 -*- described which appear open to the objection of being very 

 complicated, and of requiring very minute precautions. 



In investigating the properties of formic aldehyde, we have 

 observed that this substance gives with ammoniacal solutions of 

 silver nitrate, adherent mirrors which can be readily polished. 

 By working under suitable conditions, we have observed that the 

 greater part of the silver contained in the solutions is deposited on 

 the glass, thus avoiding residues and diminishing the cost. 



After numerous trials we have arrived at the following method. 



100 cubic centhn. are taken of a 10-per cent, solution of silver 

 nitrate to which ammonia is added drop by drop, so as just to 

 redissolve the precipitate formed at first. Care must be taken to 

 avoid an excess of ammonia, which would hinder the formation of 

 the deposit. The volume of the solution is made up to a litre by 

 distilled water, and we thus obtain what we shall call A. 



On the other hand, commercial formaldehyde of 40 per cent, is 

 diluted with distilled water so as to form a 1-per cent solution. 

 Owing to its dilution this solution, B, may be kept for some time. 



The surface of the glass is carefully polished by rubbing it with 

 chamois covered with rouge, and two volumes of A are rapidly 

 mixed with one volume of B, and the mixture is rapidly poured 

 over the glass to be coated. 



In five or six minutes, at the temperature of 15° to 19°, all the 

 silver in the solution is deposited in a brilliant layer which is 

 washed with water. It is then dried, and varnished if the surface 

 in contact with the glass is to be the reflecting surface, or 

 polished with the ordinary precautions where the layer itself is 

 to be used as in astronomical instruments. — Journal cle Physique, 

 January 1895. 



ON THE GLOW-DISCHARGE. BY A. HERZ. 



The results of this research are summed up by the author as 

 follows : — 



1. The potential gradient v in the positive uustratified glow- 

 light with constant current diminishes as the current i increases, 

 and the value of this decrease is given with sufficient accuracy by 

 the equation 



v=v -b(i-i ). 



The value 6, that is the decrease of the gradients for unit 

 increase of the current strength, decreases as the width of the tube 

 in the clear increases, and for the same width has almost the 

 same value for nitrogen and for hydrogen. 



