44 Messrs. Boys, Briscoe, and Watson on the 



such a length of time that we thought it would be interesting 

 to analyse the resulting substance. 



The residue from the action of the pure acid on the lead 

 consisted of 82 per cent, of sulphate of lead and 18 per cent, 

 of metallic lead, while that from the action of acid mixed with 

 sulphate of soda consisted of 81) per cent, of lead sulphate 

 and 11 per cent, of metallic lead. 



Although the action of the acid on the lead was initially 

 diminished by the presence of sodium sulphate, the final result 

 in this instance was rather the other way. No doubt the 

 whole of the spongy lead would have been converted into 

 sulphate if it had not been for the large amount of insoluble 

 salt which protected the remaining particles. 



This result throws light on the reason why a lead plate 

 deteriorates during a long rest. 



V. On the Measurement of Electromagnetic Radiation. By 

 C. V. Boys, A.R.S.M., F.R.S., Assistant Professor of 

 Physics at the Royal College of Science, London, A. E. 

 Briscoe, A.R.C.S., and W. Watson, A.R.C.S., B.Sc* 



[Plate V.] 



AT the time that Mr. Gregory described and exhibited his 

 " Electric Radiation Meter " |, which indicated and 

 measured electromagnetic radiation by the increase in length 

 of a fine platinum wire stretched in a glass tube in consequence 

 of the heat developed by the induced currents within it, one 

 of us expressed the view that it would be interesting to 

 confirm by some independent method the conclusion that 

 the wire was lengthened by heat, as it was not absolutely 

 certain, though exceedingly probable, that the increase in 

 length might not be apparent only and not real. 



The following is an account of experiments made by two 

 independent methods, with the view of finding whether the 

 expansion observed by Mr. Gregory was such as might be 

 expected from the induced currents. 



First Method. 



Imagine two straight wdres parallel and near together, but 

 not touching one another or any conductor. Let them be 

 placed parallel to and at a convenient distance from a cylin- 

 drical rod of the same length as either of the wires. Then, 

 if electrical oscillations are started in this rod by any well- 



* Communicated by the Physical Society ; read June 20, 1890. 

 t Phil. Mag. Jan. 1880. 



