444 PROFESSOR A, SCHUSTER AND MR. W. GANNON ON A 
time during the same time without vitiating the experiment, for as long as we assume 
Newron’s law, it does not matter whether the calorimeter rises uniformly or the 
enclosure falls uniformly or whether they both vary together. This is an important 
consideration, for unless we take very elaborate precautions, the enclosure must to a 
certain extent follow the temperature of the calorimeter. All that is requisite for a 
correct estimate of the cooling is that the rates should be accurately known at the 
beginning and ending of the second period. The change in temperature of our water 
jacket in the actual experiments was very small and sometimes inappreciable. 
When the temperature of our calorimeter was raised 1° above that of the water 
jacket, the cooling amounted to about 0°-0025 per minute; the cooling surface was 
about 725 sq. centims. ; the amount of water in the calorimeter, 1500 grams. Hence 
there was a loss of heat for each square centimetre of surface of 0:0052 unit per 
minute. In Mr. E. H. Grirrirus’ determination, where the pressure of the sur- 
rounding space was reduced to about one-hundredth atmosphere, the corresponding 
number was between 0'0028 and 0°0026, taking 300 sq. centims. as the exposed 
surface. On the other hand, his total amount of water was always less than one- 
quarter of that used by us; so that the actual cooling in his experiments for one 
degree difference was about double ours. It is interesting also to compare our 
number with that corresponding to the ioss from a cylindrical rod suspended - 
horizontally in the air and quite unprotected. Mr. Less (‘ Phil. Trans.,’ Vol. 183, 
p- 490) finds for a nickel-plated rod of about 2 centims. diameter, 0°011 gramme-degree 
per square centimetre per minute, or about twice our number. In the latter case, the 
conditions of the experiment are most favourable for great loss of heat by convection ; 
and Mr. GRIFFITHS’ experiments show that by reduction of pressure to about one-third 
of a millimetre the loss is only reduced to about one-quarter of what it is in that 
case, and one-half of what it may easily be reduced to in a calorimetric experiment at 
atmospheric pressure. It ought to be possible by properly disposed diaphragms 
to reduce considerably the effects of convection currents at atmospheric pressure and 
SO gain in a simple way the same advantage as is done by removing the air surrounding 
the calorimeter. 
We may calculate approximately that part of the loss which is due to conduction 
irrespective of convection, Taking the thermal conductivity of air to be 0°000055, 
and the distance between the calorimeter and the enclosure as 3°8 centims., we find 
that thermal conductivity alone would account for a loss of roughly 0:001 gramme- 
degree, or about one-fifth only of the actual loss. As in Mr. GRiFFrTHs’ experiments 
the enclosure was at a greater distance from his calorimeter than ours, his observed 
loss by radiation and conduction is also about five times that due to conduction of 
air alone. 
Evaporation will produce a certain amount of cooling of the calorimeter, if that is 
not perfectly enclosed; but unless the rate of evaporation changes with the 
temperature, the effect will only be a lowering of temperature by a constant quantity. 
