of Edinhurglh, Session 1883 - 84 . 
685 
deposited on it, from a solution of the nitrate. By this simple 
process I had now acquired an extremely interesting and curious 
instrument. Its powers of repelling radiant heat, if we may use 
the expression, are very remarkable. 
My first experiments with this instrument were made in the 
beginning of this month. I first wished to get the heating effect 
of the sun’s rays on this silvered thermometer in calm air ; the 
experiments were therefore made in a room, and only enough of 
the window opened to allow the sun to shine in on the thermometer, 
which was placed with its bulb in sunshine. Alongside the 
silvered thermometer was hung an ordinary thermometer, and 
readings were taken from time to time of the temperature of the 
room and of the sun-warmed thermometers. In these tests the 
silvered thermometer never rose more than one degree abov^e the 
temperature of the room, and was often only half a degree, and 
sometimes less, while the other thermometer rose from 4 to 41- 
degrees above the temperature of the room. In these experiments 
it was found extremely difficult to get the true temperature of the 
room with an ordinary thermometer, so much depended on where 
the thermometer was placed, and the amount of radiation that 
reached it, but the result given is as correct as I have been able to 
make it. Another difficulty in making these experiments is the 
extreme sensitiveness of the fine-bulbed thermometer, it is so 
constantly rising and falling nearly a degree in little more than a 
minute. To give an idea of the difficulty of getting the tempera- 
ture of the room where the experiments were made, I may mention 
the following, as it at the same time shows how much the indica- 
tions of ordinary thermometers are affected by radiation. At one 
stage in the experiments the thermometer employed for giving the 
temperature of the room was hung alongside the other thermometers, 
but provided with a shade wffiich protected it from all direct sun- 
shine. When in this position it was in the same air as the silvered 
thermometer, yet it always read higher, sometimes by nearly one 
degree ; that is, the ordinary thermometer was more heated by the 
radiant heat from the carpet, and other objects in the room on which 
the sun was shining, than the silvered bulb was by the direct rays 
of the sun added to the radiation from surrounding objects. 
The next tests with this instrument were made outside under the 
