Letter from Professor Lcdie to the Editor, 171 



It would be unnecessary to follow the rest of the experiments 

 brought forward by Mr Ritchie, which seem neither happily 

 devised, nor capable of much accuracy. 



But a very simple and unexceptionable experiment will set 

 the question at rest. I had a dift'erential thermometer, with pa- 

 rallel branches, constructed of rather large dimensions, one of 

 the balls blown as thin as possible, and the other extremely 

 thick, perhaps the fifteenth part of an inch in thickness. Into the 

 cavity of this ball, sulphuric acid, tinged with carmine, was in- 

 troduced, sufficient to fill both branches; and the tubes being- 

 united, and properly bent, the liquid was adjusted to stand 

 about the middle of the stem, under the thin ball. On placing 

 the instrument near a clear strong fire, the thin ball being more 

 quickly affected, the liquid sank rapidly in the stem, but again 

 rose gradually, and in the space of about ten minutes recovered 

 its station. There it remained, but with a slight fluctuation, 

 owing to some occasional variation in the strength of the fire, 

 or to the fluctuation in the air of the room. On withdrawing 

 this differential thermometer again, the liquid mounted swiftly 

 into the thin ball, but again subsided gradually to its stationary 

 point. 



Since both balls, then, were placed in exactly similar circum- 

 stances, it follows, that they were equally affected by the afflux 

 of heat, and that no portion of this heat had been transmitted 

 through either of them. 



When this differential thermometer was employed as a pho- 

 tometer, it indicated a different effect. Placed in the inside of 

 a room, but close to a south window at noon, the liquor always 

 mounted several degrees, a sensible portion of the light of the 

 sun being absorbed by the thick ball, while it passed without 

 interruption through the thin ball. 



I have only to add, that the instrument which Mr Ritchie 

 proposes in the same volume, as a new and peculiarly delicate 

 photometer, is only one of the vai'ious modifications of the diffe- 

 rential thermometer, which in my earlier experiments I tried 

 for measuring small quantities of light, but which I soon laid 

 aside, on finding its performance to be quite irregular and un- 

 certain. 



