924 ‘Defence of the Objections to (Marew, 
by Mr. Prevost) is that the effect depends on thé power of: the 
different surfaces in reflecting radiant heat. Of the heat radiated — 
from the thermométer 4 portion is always returned by reflection. 
A biackened surface, it is remarked, radiates much, but it reflects 
little; while it intercepts radiation or reflection from behind. A 
metallic surface radiates less, but it reflects as much as it fails to 
radiate ; hence it is inferred, that by reflecting so much heat, 
though it radiates so little, it is powerful in counteracting the fall 
of the thermometer. 
In this reasoning it appears to me that too little effect is ascribed 
to radiation, and too much to reflection. The comparative powers 
of different surfaces in producing the phenomenon of radiant heat 
show how much more influence is-due to the radiating than to the 
reflecting power; the blackened surface which reflects scarcely any 
producing, when opposed to the thermometer, the greatest heating 
effect, because it radiates most; while the metallic surface, which 
returns the largest quantity by reflection to the thermometer, still 
produces the least heating effect, because it is inferior in radiating 
power. In the experiment with radiant cold, the same difference 
of effect ought to take place; the blackened surface, though re- 
flecting little, still by its superior radiating power oaght to, produce 
the greatest heating effect, so as to counteract the fall of tem- 
perature in the thermometer; and the metallic surface, though it 
reflects best, yet emitting so little by radiation, ought to produce 
the least heating effect, and therefore admit of the greatest de- 
pression of temperature in the thermometer, all of which is the 
reverse of the fact. Jt seems to me, therefore, that the original 
argument is still just, and that, according to Prevost’s hypothesis, 
the blackened surface ought to be least powerful in producing 
radiant cold. Or if even the circumstance of its inferior reflecting 
power should so far counterbalance its superior radiating power as 
to render it equal to the other, still no cause can be assigned for its 
cooling agency being so greatly superior. “ A blackened surface,” 
says Mr. Davenport, “ radiates much it is true, but it intercepts 
an equal volume of radiation or reflection from behind; a polished 
surface radiates less, but it reflects as much as it fails to radiate.” 
Its power, therefore, ought to be the same as that of a surface 
reflecting little ; but which xadiates as much as it fails to reflect; 
that is, the power of the two surfaces ought to be the same, and 
there is no cause why the blackened surface should be so far superior 
to the other in producing cold. 
Tam Sir, yours respectfully, 
J. Murray. 
P.S. I take the liberty of pointing out a slight oversight in the 
statement in your last number (pages 43 and 44) with regard to my 
paper on Mineral Waters. It is mentioned that, in the opinion I 
had advanced of muriate of lime and sulphate of soda being 
present together in a mineral water, I had been anticipated by 
Pfaff, who had stated muriate of lime and sulphate of magnesia as 
