BY JAMES TOLSON, ESQ. 61 
temperature of 60° F., and let these surfaces be exposed to distant 
walls, WW, maintained at 59° or 1° lower than SS. 
“‘Under these circumstances the surfaces, SS, will obviously 
lose no heat by contact of cold air because the air has the same 
temperature as itself ; for a similar reason there will be no loss by 
conduction because P has the same temperature all through ; but 
SS will send out in all directions towards WW rays of radiant 
heat, which will proceed in straight lines through the air until they 
are intercepted and absorbed by the walls. The amount of heat 
thus lost will vary exceedingly with the nature of the surfaces. 
Thus according to the Table 1, which gives the loss in 
units of heat per square foot per hour, for 1° difference between S 
and W, as in this case, for building stone, this loss may be taken 
at "736 units per hour. If W had the same temperature as S, radia- 
tion would cease altogether ; if, on the other hand, the temperature 
was reversed, S being in that case 1° Jower than WW, it would 
absorb the same amount of heat as it emitted in the former case ; the 
radiating and absorbing power of bodies being equal to each other. 
‘*For ordinary atmospheric temperatures of absorbing surfaces, 
say 40° F., and sma// differences between S and W, we may admit 
that the loss of heat is simply proportional to that difference, but 
for high temperatures and great differences between S and W, the 
loss of heat is much greater, following a complicated law from 
which Dulong has given a rule that agrees very well with experi- 
ments. 
““The loss of radiant heat is not affected by the form of the 
radiant body—a cube, a square, or a cylinder, etc., will radiate the 
same amount with equal areas under the same conditions, so long 
as the body is not of such a form as #0 radiate toand from itself. 
‘““The colour of the surface seems to have no effect on the 
radiant power of bodies, at least this is true of paper and woven 
fabrics. 
“The radiation of heat is not affected by the distance of the 
absorbent. Ifthe space were a vacuum it would be quite immaterial 
whether WW were inches or millions of miles distant from S, the 
rays of heat would travel on until they were absorbed by some 
cold body that intercepted them. ‘Thus, the heat we receive from 
the sun is radiant heat that has travelled g2 millions of miles 
through space.” —Box, op. cit. 
‘* Newton was the first to enunciate his views on the cooling of 
bodies. He supposed that a heated body exposed to a certain 
cooling cause would lose at each instant a quantity of heat propor- 
tional to the excess of its temperature above that of the surrounding 
air. It was, however, soon found that this law was not exactly 
