238 



Prof. T. Carnelley and Mr. W. Mackie. 



The chief difficulty of the comparison is that of protecting the iron 

 wire from alteration when it is maintained at a red heat for some 

 time. The following Table, II, gives some observations selected at 

 random from a long series in which the zero variation of the iron did 

 not exceed 0'05 per cent. The observations were mostly taken on 

 different days ; and the resistances observed at the air temperature 

 in the intervals show the direction in which the correction for zero 

 variation should be applied, but not its amount. It is noteworthy 

 that in all cases the correction for this would tend to reduce the 

 small differences between the formula (e) and the observations. As 

 it is, the mean difference is only 0°'l C. 



It does not appear that any other equally simple and convenient 

 formula could be found representing the observations equally well. 



Treating the observations on the comparison of two platinum wires 

 in precisely the same way, we shall find for the values of the constants, 

 assuming a/3 as before for the standard wire — 



Standard wire * =0 '0034259. (3 =0 '0015290 



Compared wire a '=0 '0033332. 0' = O '0015256 



so that the values of /3 are almost identical. 



If [i=ft f we have the relation between the resistances at the same 

 temperature 



dR I dJV_ aR log R __k 

 dt I dt a'JR/' ° r log R' 



It is intended to continue the experiments at higher temperatures, 

 and to apply the platinum thermometer to other investigations. 



(< The Determination of Organic Matter in Air." By Pro- 

 fessor Thos. Carnelley, D.Sc., and Wm. Mackie, M.A., 

 University College, Dundee. Communicated by Sir H. E. 

 EOSCOE, F.R.S. Received June 10. Eead June 10, 1886. 



The only methods hitherto proposed for determining the amount of 

 organic matter in air are the two devised by the late Dr. Angus Smith 

 (" Air and Rain"). According to the first of these methods, a defi- 

 nite quantity of the air to be examined is slowly bubbled through a 

 dilute solution of potassium permanganate of known strength until it 

 is fully or considerably bleached, and in the latter case the amount of 

 undecomposed permanganate determined by oxalic acid. In the 

 second method a known volume of air is bubbled through distilled 

 water, and the latter examined for free and albuminoid ammonia by 

 Wanklyn and Chapman's process for water analysis. 



