Mr Ritchie's Observations on Leslie's Photometer. 105 



direct contradiction to the experimental results of Delaroche and 

 Berard. You must have misunderstood my meaning; as all the 

 experiments which I have performed, with transparent screens, 

 are in perfect unison with the results of these eminent philoso- 

 phers, viz. That invisible caloric, radiating from an elevated 

 source, will, like light, permeate very thin plates of glass, 

 whilst it is completely intercepted by thicker plates. I have 

 lately performed numerous experiments with transparent 

 screens, of such extreme tenuity, as to allow the instantaneous 

 passage of heat, radiating from a source at an inferior temper- 

 ature, with the same facility and copiousness as light through 

 thicker plates. Now the glass case, which surrounds the in- 

 strument, is sufficiently thick to arrest the progress of heat 

 unaccompanied with light, and consequently the effect upon 

 the instrument will be the same, as if the case were perfectly 

 opaque. With regard to the apparent discrepancy between 

 the results of Dr Christison and Dr Turner, and those which 

 I obtained, permit me to offer a few remarks. In the porta- 

 ble photometer, (the one most probably employed by Drs C. 

 and T.) more heat will undoubtedly have an influence on the 

 higher ball, though both balls were perfectly transparent ; 

 for the caloric, conducted through the glass-case, will first 

 combine with the film of air adjacent to the interior surface of 

 the cylinder, and cause its immediate expansion. The air, 

 thus rendered specifically lighter, will mount to the top of the 

 cylinder, and accumulate round the.Jiigher ball. The tem- 

 perature of the air in the higher ball will thus receive a greater 

 elevation than that in the lower one, and cause a depression of 

 the fluid in the stem. When both balls. are, however, placed 

 on the same level, (as in the stationary photometer,) no such 

 effect can take place, since mere caloric is equally absorbable 

 by white and black surfaces. In the portable form, the in- 

 strument will be differently affected by mere heat, according 

 to the distance between the two balls. 



The portable photometer is also subject to several inaccu- 

 racies besides those already mentioned. As the end of the 

 cylindrical case is closed by the blow-pipe, it is almost impos- 

 sible to preserve it of the same thickness and transparency 

 with the rest of the tube. Hence the quantity of light, which 



