Professor Forbes's Researches on Heat. Fourth Series. 109 



reality a worse conductor than this metal, a contradiction 

 which has been maintained by Prof. Poggendorff; but it is, 

 that iron, when in galvanic action, retains a more bright and 

 metallic surface than copper does in like circumstances. This 

 is also the case with all metals possessing the apparently ano- 

 malous property of being positive to a metal A, when asso- 

 ciated with it as a galvanic pair; but more highly negative, or 

 at least giving a stronger electric current than this metal A 

 would, if associated with a metal B, still more positive than 

 either of the former. I may call the attention of experimenters 

 to the ceconomy and power of the iron-zinc battery, which, by 

 a simple contrivance to ensure a regular supply of acid, can 

 be made as constant in its action as would be desired for or- 

 dinary purposes. 



Compared with the copper-zinc battery, the following results 

 show its power and ceconomy : — 



A small ten pair iron-zinc tested against a similar sized 

 copper-zinc battery : an equal measure of dilute acid was 

 poured into each battery; both were employed in decompo- 

 sing water until all action ceased. The iron-zinc produced, 

 by the expenditure of this acid, four cubic inches of mixed 

 gases in 104 minutes. The copper-zinc produced, by a like 

 expenditure of acid, only one and a half cubic inches of 

 mixed gases, and this required 125 minutes. The expendi- 

 ture of zinc is also much less when associated with iron than 

 with copper, and this in the proportion of twenty to twenty- 

 seven. 



Prof. Poggendorff states, that it is necessary to give a zinc- 

 platina battery three times as large a surface as that of a zinc- 

 iron one to produce equal effects; and as zinc-platina has 

 hitherto been considered the best galvanic arrangement, with 

 the exception of DanielPs, the ceconomy of the zinc-iron bat- 

 tery is manifest. 



I am, &c, 



Martyn J. Roberts. 



XXI. Researches on Heat. — Fourth Series. On the Effect of 

 the Mechanical Texture of Screens on the immediate trans- 

 mission of Radiant Heat. By James D. Forbes, Esy., 

 F.R.SS. L. S$ E., Professor of Natural Philosophy in the 

 University of Edinburgh. 



[Continued from p. 81, and concluded.] 

 26. A MORE minute analysisof the influenceof surface upon 

 •£*- heat is what we now propose. And three questions 

 present themselves for immediate solution: — (1.) If deficiency 



