72 Intelligence and Miscellaneous Articles. 



silver salt) is wholly without developing-power, an opportunity 

 offered itself of testing the question. And it proved that a solu- 

 tion obtained by adding one of disodic phosphate to one of ferrous 

 sulphate until a permanent precipitate began to form, undoubtedly 

 possessed developiug-powers, though in a less degree. 



The number of ferrous salts capable of developing the latent 

 image is very considerable. Singular anomalies are often shown : 

 a given salt prepared in one way may develop, while prepared in 

 another it may have no such power. Nor is it possible to form an 

 opinion beforehand as to whether a given compound of ferrous 

 oxide will exhibit this power or not : compounds nearly allied do 

 not exhibit analogies in this respect. For example, ferrous phos- 

 phate and ferrous metaphosphate are active developers, while fer- 

 rous pyrophosphate has no similar power. 



Among other ferrous salts possessing more or less developing- 

 power may be mentioned ferrous hyposulphite (hydrosulphate), 

 am m onio-chloride, acetate, antimonio-tartrate, &c. Ferrous for- 

 miate, which might naturally be expected to be a powerful deve- 

 loper, is almost, though not entirely, destitute of the property. 

 The most active agents found were ferrous borate, phosphate, sul- 

 phite, and oxalate, respectively dissolved, the phosphate in neutral 

 ammonium oxalate, the others in neutral potassium oxalate. — Silli- 

 man's American Journal. June 1880. 



NOTE ON THE CONSTRUCTION OF GUARD-RING ELECTROMETERS. 

 BY GEORGE FRANCIS FITZGERALD, M.A., F.T.C.D. 

 Guard-ring electrometers have usually been constructed with an 

 aluminium disk, for the sake of lightness, surrounded by a guard- 

 ring of brass. It is essential for the accuracy of the calculation 

 of the absolute values of capacity and quantity made with them 

 that the electricity should be as uniformly distributed as possible 

 on the surface of the disk and guard. It is for the sake of produ- 

 cing a uniform distribution on the disk that the guard is added. 

 Hence any arrangement which disturbs this uniformity of distri- 

 bution is to be avoided. Now, whether the contact of dissimilar 

 metals in itself produces an appreciable difference of potential be- 

 tween them, or whether it is the air near different metals that is at 

 different potentials, there is no doubt that when the plates of an 

 accumulator are of different metals there is an appreciable accumu- 

 lation of electricity upon them. Consequently in the guard-ring 

 electrometer the distribution of electricity on the aluminium disk 

 cannot be the same as on the brass guard connected with it. It 

 might seem as if the other plate should be of the same material ; 

 but as it is generally easy to apply the differential method of mea- 

 surement, a constant, even though unknown, difference of potential 

 between the plates is of no consequence. — Scientific Proceedings of 

 the Royal Dublin Society, Jan. 19th, 1880. 



