Radiation of Heat by Gases and Vapours. 171 



was thus fastened on the green silk; some iron compound had 

 been used in the dyeing of it ; and to this the deviation of my 

 needle from zero was manifestly due. 



I had the green coating removed and the wire overspun with 

 white silk, clean hands being used in the process. A perfect 

 galvanometer is the result. The needle, when released from the 

 action of a current, returns accurately to zero, and is perfectly 

 free from all magnetic action on the part of the coil. In fact 

 while we have been devising agate plates and other elaborate 

 methods to get rid of the great nuisance of a magnetic coil*, the 

 means of doing so are at hand. Nothing is more easy to be 

 found than diamagaetic copper wire. Oat of eleven specimens, 

 four of which were furnished by Mr. Becker, and seven taken at 

 random from our laboratory, nine were found diamagnetic and 

 only two magnetic. 



Perhaps the only defect of those fine instruments with which 

 Du Bois Raymond conducts his admirable researches in animal 

 electricity is that above alluded to. The needle never comes to 

 zero, but is drawn to it by a minute magnet. This defect may 

 be completely removed. By the substitution of clean white silk 

 for green, however large the coil may be, the compensator may 

 be dispensed with, and a great augmentation of delicacy secured. 

 The instrument will be rendered suitable for quantitative mea- 

 surements ; effects which are now beyond the reach of experi- 

 ment will be rendered manifest; while the important results 

 hitherto established will be obtained with a fraction of the length 

 of wire now in usej\ 



§ 3. Our present knowledge of the deportment of liquids and 

 solids, would lead to the inference that, if gases and vapours 

 exercised airy appreciable absorptive power on radiant heat, the 

 absorption would make itself most manifest on heat emanating 

 "from an obscure source. But an experimental difficulty occurs 

 at the outset in dealing with sueh heat. How must we close 

 the receiver containing the gases through which the calorific 

 rays are to be sent ? Melloni found that a glass plate one-tenth 

 of an inch in thickness intercepted all the rays emanating from 

 a source of the temperature of boiling water, and fully 94 per 

 cent, of the rays from a source of 400° Centigrade. Hence a tube 

 closed with glass plates would be scarcely more suitable for the 

 purpose now under consideration, than if its ends were stopped 

 by plates of metal. 



* See Melloni upon this subject, Thermochrose, pp. 31-33. 



t Mr. Becker, to whose skill and intelligence I have been greatly in- 

 debted, furnished me with several specimens of wire of the same fineness as 

 that used by Du Bois Raymond, some covered with green silk and others 

 with white. The former were invariably attracted, the latter invariably 

 repelled. In all cases the naked wire was repelled. 



N2 



