No. XXIL] APPENDIX, 303 



specific gravity 1130 is called unity, and the interval between that part and 

 1360 is divided into 144 parts. By this division every degree represents 

 one grain of zinc dissolved in 1000 grains of bulk of the fluid. The 

 opposite side of the scale, between the same parts, is divided into 60 parts, 

 each of which is, for every 1000 grains of bulk in the fluid, about YOOO ^ 

 an inch in the thickness for every superficial inch of surface, upon which 

 the copper is reduced in the precipitating trough. In this division a little 

 allowance has been made for some local action of the zinc. By this 

 instrument we really weigh the zinc which has entered into combination 

 with the oxygen of the water in which it is subsequently dissolved. By the 

 attraction between the zinc and the element of the water the power is 

 produced wherewith the plates of the bank-notes are made, and this attrac- 

 tion differs not in kind from the attraction between the coals and air in 

 the act of combustion which gives us the power in the steam-engine. In 

 the electro-metallurgic battery, however, is perhaps observable the first 

 instance of the estimation of the primary change of matter, to determine 

 the amount of work actually performed. In the steam-engine the coals 

 burnt will not necessarily give us a satisfactory clue to the work done ; 

 and even in the animal, the most perfect of all machines, the food the 

 soldier eats will not indicate the number of miles traversed, or of the 

 enemy killed. 



In an application of the battery-meter we have an illustration of a law 

 which governs all physical phenomena. Without a change of matter we 

 can have no physical force ; and all physical force is referable to a corre- 

 sponding change of matter. In our electro-metallurgic apparatus we obtain 

 an effect equal to the original change of matter within a very trifling per- 

 centage, a result which must be regarded as a glorious triumph of human 

 improvement. If by the use of the battery-meter these great laws are 

 popularized, and lead to a more universal reference of effect to cause, it 

 will amply repay any little trouble which has been bestowed upon it. 



To contain the battery with its charge, we generally employ the best 

 salt-glazed stoneware. Strange as it may seem, no form of earthenware 

 permanently resists the attacks of the metallic saline solution. They pass 

 into the innermost texture of the material, and, even with vessels for 

 holding writing ink, disintegration eventually ensues. Upon the whole, 

 earthenware is preferable to glass, because it is less brittle, and I trust that 

 the mention of the subject may lead some member of the Society to produce 

 a cheap material, as impermeable as glass, and as durable as pottery. 



At the Bank of England we generally find it convenient to employ 

 parallelepiped- shaped vessels. Those made of mahogany and lined with 

 gutta-percha are convenient and economical. For most of our purposes, 

 we use the vertical trough, because the subject can be readily inserted and 

 removed for inspection. For rapid deposition we employ the horizontal 

 trough, in which the subject is placed at the bottom, and the copper pole 

 above. In the use of this apparatus some refined chemical laws are in- 

 volved. In the first place, sulphate of copper possesses a low diffusive 

 power, and is carried, by virtue of that property, so slowly through the 

 fluid, that if we relied upon it failure would surely attend our labour. 

 Secondly, the saturated solution of sulphate of copper formed at the 

 positive pole is so heavy that it descends from the place of its formation, 

 like a cataract, to the bottom of the vessel. Lastly, the part of the solution 



