of Edinburgh , Session 1885 - 86 . 639 
it is only at the intermediate points that they have a disturbing 
influence. 
Cotton Wool , Medium Quality. 
1. 
2. 
3. 
4. 
Air. 
Case. 
Difference. 
Protection. 
o 
o 
-rJi 
32°‘6 
18° *6 
None. 
13° '8 
13° *8 
o 
o 
6 
1 Thickness. 
The above table shows the result with cotton wool. The kind 
known as medium quality was used, the sheets of which are not so 
thick as those of a finer kind. For supporting the cotton wool in 
its place inside the case, a wire-cloth cage was used, its inside 
being lined with one thickness of the wool from which the 
shin had been previously removed. This extremely feeble-looking 
protection, as will be seen from the table, acted perfectly ; the 
thermometer when protected by it was quite unaffected by the hot 
case. A number of readings were taken when the case was heated 
to 100°, and when cooled with water, but not the slightest difference 
could be detected. Without any gas to assist the draught, the 
temperature rose 0 o, 2 above the temperature of the air. 
It will be noticed that a very thin covering of cotton wool or 
muslin, when employed in the manner here described, is quite 
sufficient to stop the advance of all heat from the warm walls of the 
case surrounding the thermometer; and further, it is interesting to 
notice that the amount of muslin and cotton wool that stops all heat 
is far from being able to stop all light. A good deal of light can 
come through a sheet of cotton wool, or twelve thicknesses of muslin. 
This peculiar action reminds us of the somewhat similar action of 
glass and other substances, which pass the rays of one end of the 
spectrum, while they stop those of the other. 
When we consider the action of muslin used in this manner 
to check radiation, we see it is one that has been long well 
known. Gardeners are well acquainted with the fact that any 
cloth, however thin, spread over plants, is a great protection to them 
on frosty nights. The action of the cloth on these occasions is 
exactly the same as in the thermometer case ; the thin cloth checks 
radiation, and prevents the heat from passing into space. The cloth 
itself is but little cooled by radiation, as it is constantly receiving heat 
