140 Drs Turner and Christison on the effects of 



the screen acting as a secondary source t)f heat. Mr Leslie 

 has proved, both by experiment and reasoning, that, if an 

 opaque screen be placed between the heated body and the reflec- 

 tor, a striking difference of effect will be observed with every 

 remove of the screen from the heated body. " At each remove," 

 says he, "the impression upon the focal ball will regularly di- 

 minish ; insomuch, that, when the screen has gained a position 

 one foot advanced from the canister, and consequent!} two 

 feet from the reflector, it will not exceed the thirtieth part of 

 the full effect." * Now we have endeavoured to prove by nu- 

 merous experiments, that the effect on the thermometer is 

 nearly the same, whether the thin liquid screen be placed near 

 the heated ball or the differential thermometer. If this ex- 

 periment be correct, the theory of aerial pulsations cannot 

 possibly be the true theory of radiant heat. I have request- 

 ed Professor Leslie to examine the delicate experiments to 

 which I have alluded ; and I am convinced, that, should they 

 be correct, he, from his love of philosophical truth, will be the?^ 

 very first to abandon a theory which, though no longer te- 

 nable, has in his hands been the means of advancing this de- 

 partment of physical science. 



Aet. XXIV. — On the Effects of the Poisonous Gases on Vege- 

 tables. By Edward Turner, M. D. F. R. S. E. Professor 

 of Chemistry in the University of London ; and Robert 

 Christison, M. D. Professor of Medical Jurisprudence 

 arid Police in the University of Edinburgh, &c.-f- 



Having been called on last July to give evidence before the 

 Jury Court, in the case of Hart against Taylor, relative to 

 the effects of a black-ash manufactory on the vegetation in its 

 neighbourhood, and having been unable to find any distinct 

 facts recorded respecting the influence on vegetable life of 

 the gases disengaged in the preparation of that article, we were 

 induced to make some experiments on the subject, which were 

 originally intended for our private satisfaction only. But we 

 have since understood, that the case in which we were concern- 



• Inquiry f p. 30. ^ » 



\ From the Edin. Med. and Surg. Journal, No. 93. . 



