{ 247 J 
LU}. On Dr. Murray’s Opposition to Professor PREVOST’S 
Theory of Radiant Caloric ; on Electrical Phenomena, and 
on Sir H. Davy’s Safe Lamp. 
To Mr. Tilloch. 
Sir,— Leis Murray has opposed the theory of Prevost on 
Radiant Heat. He (Dr. M.) says that the polished metallic 
surface of the canister containing a freezing mixture, when 
placed in the focus of a mirror, opposing another holding a ther- 
inometer, should depress the temperature more thanthe Ldackened 
eide of the canister, if this theory be truc ; but that the contrary 
is the fact. 
It is accepted that a blackened surface radiates heat more 
copiously than when the metallic surface is unimpaired ; and it 
js inferred by the Doctor fram this circumstance, that heat ra- 
diating in greater quantity from the canister by means of the 
black surface, should compensate more largelythan the uncovered 
surface for the loss of temperature sustained by the thermometer 
in the opposing reflector. Experiment, however, teaches that 
the thermometer is less depressed by the clean metallic than the 
painted surface, 7. e. there is a greater decrement of temperature 
by the dark surface than by that which has an unclouded lustre. 
it must however be admitted, that the various colours absorb 
heat differently; black surfaces exceeding them in this respect. 
Hence white unpolished surfaces absorb heat slower than those 
that are blackened. The decrement of temperature may be ac- 
counted for by presuming that the facilities of absorption are 
in a more exalted ratio than the powers of radiation. 
Mr. WALKER on Electrical Phenomena. 
Your indefatigable correspondent Mr. W., amon his quota- 
¢ r 5] 
tions, refers to some experiments which do not tend to support the 
views of which he is the advocate. When aqueous vapour, it is 
stated, passes through an ignited porcelain tube (lined with char- 
coal), hydrogen is formed. This is not the case—A mixture of car= 
bonic acid and hydrogen, as might be naturally expected, passes 
over into the recipient, forming hydro-carlbonate. It is dificult 
to believe that pure carbonic acid gas can be obtained by passing 
steam through a tube lined with carbonate barytes. But grant 
it true, 1 can conceive that the quantum of heat employed may 
serve todetach carbonic acid gas from the carbonate barytes, and 
that this aériform fluid may be found in the inverted vessel on 
the pneumatic cistern — no danbt im company with aqueous 
vapour—That carbonic acid gas is formed in undimited quantity 
from a limited portion of carbonate barytes is incredible. Mr. 
Walker might as well adduce the experiment as a proof that 
Q4 water 
